fix(telegram): address PR review comments

- Export pickFirstExistingAgentId and use it to validate topic agentId
- Properly update mainSessionKey when overriding route agent
- Fix docs example showing incorrect session key for topic 3

Fixes issue where non-existent agentId would create orphaned sessions.
Fixes issue where DM topic replies would route to wrong agent.

src/routing/resolve-route.ts

@@ -143,7 +143,7 @@ function resolveAgentLookupCache(cfg: OpenClawConfig): AgentLookupCache {
   return next;
 }
 
-function pickFirstExistingAgentId(cfg: OpenClawConfig, agentId: string): string {
+export function pickFirstExistingAgentId(cfg: OpenClawConfig, agentId: string): string {
   const lookup = resolveAgentLookupCache(cfg);
   const trimmed = (agentId ?? "").trim();
   if (!trimmed) {