test: add unit tests for global connection pool (Issue #778) (#780)

* test: add unit tests for global connection pool (Issue #778)

- Add TestLifespanFunction class with 9 tests for lifespan management:
  - PostgreSQL/MongoDB pool initialization success/failure
  - Cleanup on shutdown
  - Skip initialization when not configured

- Add TestGlobalConnectionPoolUsage class with 4 tests:
  - Using global pools when available
  - Fallback to per-request connections

- Fix missing dict_row import in app.py (bug from PR #757)

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/server/app.py

@@ -17,6 +17,7 @@ from langgraph.checkpoint.mongodb import AsyncMongoDBSaver
 from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
 from langgraph.store.memory import InMemoryStore
 from langgraph.types import Command
+from psycopg.rows import dict_row
 from psycopg_pool import AsyncConnectionPool
 
 from src.config.configuration import get_recursion_limit