fix(server): make observability migration retry-safe

[abhinav-galileo]

Summary

• Make the observability namespace Alembic migration tolerate partially-applied schema state.
• Skip adding namespace_key when it already exists.
• Rewrite the event primary key only when it is not already namespace-scoped.
• Add regression tests for partial migration retries before Alembic stamps the revision.

Testing

• env -u AGENT_CONTROL_DB_URL -u DATABASE_URL -u DB_URL AGENT_CONTROL_DB_HOST=localhost AGENT_CONTROL_DB_PORT=5432 AGENT_CONTROL_DB_USER=agent_control AGENT_CONTROL_DB_PASSWORD=agent_control AGENT_CONTROL_DB_DATABASE=agent_control_test AGENT_CONTROL_DB_DRIVER=psycopg uv run --package agent-control-server pytest server/tests/test_data_model_v1_alembic_migration.py -q
• uv run ruff check tests/test_data_model_v1_alembic_migration.py alembic/versions/b6f4c2d8e9a1_namespace_observability_events.py from server/
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!