test: cover production-critical paths

[QueryPlanner]

Summary

Make the 100% branch-coverage gate include production-critical startup, agent,
observability, and memory paths. These modules are currently omitted from coverage,
so the reported total does not measure several of the highest-risk behaviors.

Priority

P1

Level of Effort

Medium (1 day)

Scope

• Add tests for server startup and shutdown lifecycle.
• Cover liveness/readiness and configuration failures.
• Cover agent and app construction with real ADK objects.
• Cover observability configuration and exporter failure behavior.
• Cover memory configuration and all memory tools.
• Remove production modules from the coverage omit list.

Passing Criteria

• server.py, agent construction, observability, memory configuration, and memory
  tools are not excluded from coverage.
• The full suite reports 100% statement and branch coverage across those modules.
• Tests use real internal classes and mock only external network, model, filesystem,
  or persistence boundaries.
• Startup and teardown are exercised independently from module-import side effects.
• CI runs the exact same coverage command documented for local development.

Sources

• Current coverage omissions
• Current server test only checks one constructor argument
• Current application lifespan