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

src/config/types.telegram.ts

@@ -187,6 +187,8 @@ export type TelegramTopicConfig = {
   systemPrompt?: string;
   /** If true, skip automatic voice-note transcription for mention detection in this topic. */
   disableAudioPreflight?: boolean;
+  /** Route this topic to a specific agent (overrides group-level and binding routing). */
+  agentId?: string;
 };
 
 export type TelegramGroupConfig = {