fix(webchat): fetch full sidebar content for truncated history

Add a bounded `chat.message.get` gateway method so Control UI can fetch one display-normalized transcript message by id when an assistant history preview was truncated. Keep `chat.history` lightweight, reject oversized/hidden/missing rows with explicit unavailable reasons, and wire the WebChat side reader to request full content only for visible truncated assistant messages.

Also refresh the generated Swift gateway protocol models and document the new assistant-message side-reader behavior.

Closes #84651.
Related #53242.

Co-authored-by: NianJiuZst <3235467914@qq.com>

docs/gateway/protocol.md

@@ -442,6 +442,7 @@ enumeration of `src/gateway/server-methods/*.ts`.
     - `sessions.reset`, `sessions.delete`, and `sessions.compact` perform session maintenance.
     - `sessions.get` returns the full stored session row.
     - Chat execution still uses `chat.history`, `chat.send`, `chat.abort`, and `chat.inject`. `chat.history` is display-normalized for UI clients: inline directive tags are stripped from visible text, plain-text tool-call XML payloads (including `<tool_call>...</tool_call>`, `<function_call>...</function_call>`, `<tool_calls>...</tool_calls>`, `<function_calls>...</function_calls>`, and truncated tool-call blocks) and leaked ASCII/full-width model control tokens are stripped, pure silent-token assistant rows such as exact `NO_REPLY` / `no_reply` are omitted, and oversized rows can be replaced with placeholders.
+    - `chat.message.get` is the additive bounded full-message reader for a single visible transcript entry. Clients pass `sessionKey`, optional `agentId` when the session selection is agent-scoped, plus a transcript `messageId` previously surfaced through `chat.history`, and the Gateway returns the same display-normalized projection without the lightweight history truncation cap when the stored entry is still available and not oversized.
 
   </Accordion>
 

docs/web/control-ui.md

@@ -167,6 +167,7 @@ Activity entries keep only sanitized summaries and redacted, truncated output pr
     - Chat uploads accept images plus non-video files. Images keep the native image path; other files are stored as managed media and shown in history as attachment links.
     - Re-sending with the same `idempotencyKey` returns `{ status: "in_flight" }` while running, and `{ status: "ok" }` after completion.
     - `chat.history` responses are size-bounded for UI safety. When transcript entries are too large, Gateway may truncate long text fields, omit heavy metadata blocks, and replace oversized messages with a placeholder (`[chat.history omitted: message too large]`).
+    - When a visible assistant message was truncated in `chat.history`, the side reader can fetch the full display-normalized transcript entry on demand through `chat.message.get` by `sessionKey`, active `agentId` when needed, and transcript `messageId`. If the Gateway still cannot return more, the reader shows an explicit unavailable state instead of silently repeating the truncated preview.
     - Assistant/generated images are persisted as managed media references and served back through authenticated Gateway media URLs, so reloads do not depend on raw base64 image payloads staying in the chat history response.
     - When rendering `chat.history`, the Control UI strips display-only inline directive tags from visible assistant text (for example `[[reply_to_*]]` and `[[audio_as_voice]]`), plain-text tool-call XML payloads (including `<tool_call>...</tool_call>`, `<function_call>...</function_call>`, `<tool_calls>...</tool_calls>`, `<function_calls>...</function_calls>`, and truncated tool-call blocks), and leaked ASCII/full-width model control tokens, and omits assistant entries whose whole visible text is only the exact silent token `NO_REPLY` / `no_reply` or the heartbeat acknowledgement token `HEARTBEAT_OK`.
     - During an active send and the final history refresh, the chat view keeps local optimistic user/assistant messages visible if `chat.history` briefly returns an older snapshot; the canonical transcript replaces those local messages once the Gateway history catches up.

docs/web/webchat.md

@@ -24,6 +24,7 @@ Status: the macOS/iOS SwiftUI chat UI talks directly to the Gateway WebSocket.
 
 - The UI connects to the Gateway WebSocket and uses `chat.history`, `chat.send`, and `chat.inject`.
 - `chat.history` is bounded for stability: Gateway may truncate long text fields, omit heavy metadata, and replace oversized entries with `[chat.history omitted: message too large]`.
+- When a visible assistant message was truncated in `chat.history`, Control UI can open a side reader and fetch the full display-normalized entry on demand through `chat.message.get` without increasing the default history payload.
 - `chat.history` follows the active transcript branch for modern append-only session files, so abandoned rewrite branches and superseded prompt copies are not rendered in WebChat.
 - Compaction entries render as an explicit compacted-history divider. The divider explains that the compacted transcript is preserved as a checkpoint and links to the Sessions checkpoint controls, where operators can branch or restore from that compacted view when their permissions allow it.
 - Control UI remembers the backing Gateway `sessionId` returned by `chat.history` and includes it on follow-up `chat.send` calls, so reconnects and page refreshes continue the same stored conversation unless the user starts or resets a session.
@@ -54,6 +55,7 @@ WebChat has two separate data paths:
 - Harnesses that require visible replies through `tools.message` still use WebChat as a current-run internal source reply sink. A targetless `message.send` from that active WebChat run is projected into the same chat and mirrored to the session transcript; WebChat does not become a reusable outbound channel and never inherits `lastChannel`.
 - WebChat injects assistant transcript entries only when the Gateway owns a displayed message outside a normal embedded agent turn: `chat.inject`, non-agent command replies, aborted partial output, and WebChat-managed media transcript supplements.
 - `chat.history` reads the stored session transcript and applies WebChat display projection. If live assistant text appears during a run but disappears after history reload, first check whether the raw JSONL contains the assistant text, then whether `chat.history` projection stripped it, then whether the Control UI optimistic-tail merge replaced local delivery state with the persisted snapshot.
+- `chat.message.get` uses the same transcript branch and display projection rules as `chat.history`, including active-agent scoping, but targets one transcript entry by `messageId` and returns an honest unavailable reason when the full content can no longer be returned.
 
 Normal agent-run final answers should be durable because the embedded runtime writes the assistant `message_end`. Any fallback that mirrors a delivered final payload into the transcript must first avoid duplicating an assistant turn that the embedded runtime already wrote.