[Bug]: Bare UUIDs return thread_not_found for MCP-created (mcp:<uuid>) threads #22401

What happens

Threads created through MCP (t3_thread_launch, create_threads) get IDs like mcp:<uuid>. Passing the bare <uuid> to t3_thread_read or t3_thread_send returns thread_not_found, and the error doesn't hint at the prefix.

Agents often copy the UUID without its prefix from logs, handoff files or other tool output, so this fails repeatedly in multi-thread orchestrations even when the full ID is in the agent's context.

Related, in the other direction: #19639 (context block gives thread_<uuid> while tools want the bare UUID).

Expected

Either form resolves, or the error points at the right form.

Suggestion

• Resolve a bare UUID to the mcp: thread when exactly one thread matches.
• Otherwise make thread_not_found say "did you mean mcp:<uuid>?".