debug: log statement_timeout + connection pid on session checkout (#9472)

* debug: log statement_timeout + connection pid on every session checkout

Temporary instrumentation to diagnose why some PlanetScale connections
have statement_timeout=5s while others have 0 (disabled).

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Co-Authored-By: Letta <noreply@letta.com>

* debug: log statement_timeout on every checkout, not just non-zero

🤖 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: rollback implicit transaction from debug query

The SELECT implicitly begins a transaction, causing "A transaction is
already begun" errors for code that calls session.begin() explicitly.

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Co-Authored-By: Letta <noreply@letta.com>

---------

Co-authored-by: Letta <noreply@letta.com>

letta/server/db.py

@@ -3,7 +3,7 @@ import uuid
 from contextlib import asynccontextmanager
 from typing import AsyncGenerator
 
-from sqlalchemy import NullPool
+from sqlalchemy import NullPool, text
 from sqlalchemy.ext.asyncio import (
     AsyncEngine,
     AsyncSession,
@@ -88,6 +88,10 @@ class DatabaseRegistry:
             try:
                 async with async_session_factory() as session:
                     try:
+                        result = await session.execute(text("SELECT pg_backend_pid(), current_setting('statement_timeout')"))
+                        pid, timeout = result.one()
+                        logger.warning(f"[stmt_timeout_debug] pid={pid} statement_timeout={timeout}")
+                        await session.rollback()
                         yield session
                         await session.commit()
                     except asyncio.CancelledError: