fix(signal): fall back to toolContext.currentMessageId for reactions

Signal reactions required an explicit messageId parameter, unlike
Telegram which already fell back to toolContext.currentMessageId.
This made agent-initiated reactions fail on Signal because the
inbound message ID was available in tool context but never used.

- Destructure toolContext in Signal action handler
- Fall back to toolContext.currentMessageId when messageId omitted
- Update reaction schema descriptions (not Telegram-specific)
- Add tests for fallback and missing-messageId rejection

Closes #17651

src/agents/tools/message-tool.ts

@@ -242,14 +242,14 @@ function buildReactionSchema() {
     messageId: Type.Optional(
       Type.String({
         description:
-          "Target message id for reaction. For Telegram, if omitted, defaults to the current inbound message id when available.",
+          "Target message id for reaction. If omitted, defaults to the current inbound message id when available.",
       }),
     ),
     message_id: Type.Optional(
       Type.String({
         // Intentional duplicate alias for tool-schema discoverability in LLMs.
         description:
-          "snake_case alias of messageId. For Telegram, if omitted, defaults to the current inbound message id when available.",
+          "snake_case alias of messageId. If omitted, defaults to the current inbound message id when available.",
       }),
     ),
     emoji: Type.Optional(Type.String()),