feat(telegram): add per-topic agent routing for forum groups [AI-assisted]

This feature allows different topics within a Telegram forum supergroup to route
to different agents, each with isolated workspace, memory, and sessions.

Key changes:
- Add agentId field to TelegramTopicConfig type for per-topic routing
- Add zod validation for agentId in topic config schema
- Implement routing logic to re-derive session key with topic's agent
- Add debug logging for topic agent overrides
- Add unit tests for routing behavior (forum topics + DM topics)
- Add config validation tests
- Document feature in docs/channels/telegram.md

This builds on the approach from PR #31513 by @Sid-Qin with additional fixes
for security (preserved account fail-closed guard) and test coverage.

Closes #31473

docs/channels/telegram.md

@@ -444,7 +444,29 @@ curl "https://api.telegram.org/bot<bot_token>/getUpdates"
     - message sends omit `message_thread_id` (Telegram rejects `sendMessage(...thread_id=1)`)
     - typing actions still include `message_thread_id`
 
-    Topic inheritance: topic entries inherit group settings unless overridden (`requireMention`, `allowFrom`, `skills`, `systemPrompt`, `enabled`, `groupPolicy`).
+    Topic inheritance: topic entries inherit group settings unless overridden (`requireMention`, `allowFrom`, `skills`, `systemPrompt`, `enabled`, `groupPolicy`, `agentId`).
+
+    **Per-topic agent routing**: Each topic can route to a different agent by setting `agentId` in the topic config. This gives each topic its own isolated workspace, memory, and session. Example:
+
+    ```json5
+    {
+      channels: {
+        telegram: {
+          groups: {
+            "-1001234567890": {
+              topics: {
+                "1": { agentId: "main" },      // General topic → main agent
+                "3": { agentId: "zu" },        // Dev topic → zu agent
+                "5": { agentId: "coder" }      // Code review → coder agent
+              }
+            }
+          }
+        }
+      }
+    }
+    ```
+
+    Each topic then has its own session key: `agent:main:telegram:group:-1001234567890:topic:3`
 
     Template context includes:
 
@@ -752,8 +774,10 @@ Primary reference:
   - `channels.telegram.groups.<id>.systemPrompt`: extra system prompt for the group.
   - `channels.telegram.groups.<id>.enabled`: disable the group when `false`.
   - `channels.telegram.groups.<id>.topics.<threadId>.*`: per-topic overrides (same fields as group).
+  - `channels.telegram.groups.<id>.topics.<threadId>.agentId`: route this topic to a specific agent (overrides group-level and binding routing).
   - `channels.telegram.groups.<id>.topics.<threadId>.groupPolicy`: per-topic override for groupPolicy (`open | allowlist | disabled`).
   - `channels.telegram.groups.<id>.topics.<threadId>.requireMention`: per-topic mention gating override.
+  - `channels.telegram.direct.<id>.topics.<threadId>.agentId`: route DM topics to a specific agent (same behavior as forum topics).
 - `channels.telegram.capabilities.inlineButtons`: `off | dm | group | all | allowlist` (default: allowlist).
 - `channels.telegram.accounts.<account>.capabilities.inlineButtons`: per-account override.
 - `channels.telegram.commands.nativeSkills`: enable/disable Telegram native skills commands.