Lazy-load Alembic in db_manager imports

[hkc-8010]

What this PR does

Avoids eagerly importing Alembic when airflow.utils.db / airflow.utils.db_manager are imported.

Today, airflow.utils.db_manager imports from alembic import command at module import time. Because airflow.utils.db imports RunDBManager, that path loads Alembic during a normal import and emits the noisy alembic.runtime.plugins INFO lines like:

setup plugin alembic.autogenerate.schemas
setup plugin alembic.autogenerate.tables
setup plugin alembic.autogenerate.types
setup plugin alembic.autogenerate.constraints
setup plugin alembic.autogenerate.defaults
setup plugin alembic.autogenerate.comments

This PR defers importing alembic.command until the DB migration operations actually need it.

Why

This keeps normal Airflow imports quiet without suppressing logs globally and without changing Alembic behavior during real migration operations.

Related issue

Closes #65652

Testing

• pytest airflow-core/tests/unit/utils/test_db_manager.py -q
• Breeze Python 3.13 / Postgres:
  • python -m pytest tests/unit/utils/test_db_manager.py -q
• Verified the original repro is clean in Breeze:
  • import airflow.utils.db_manager
  • import airflow.utils.db

[hkc-8010]

A bit of extra validation context from local repro/testing:

• I reproduced the noisy alembic.runtime.plugins output in OSS Airflow 3.2.0, not just in Astro Runtime.
• In OSS, plain import airflow stayed clean in my tests, but these import paths emitted the setup plugin alembic.autogenerate.* lines:
  • import airflow.utils.db
  • import airflow.utils.db_manager
  • from airflow.utils.db_manager import RunDBManager
• I validated the fix in Breeze with Python 3.13 + Postgres:
  • python -m pytest tests/unit/utils/test_db_manager.py -q -> 15 passed
  • import airflow.utils.db_manager -> clean
  • import airflow.utils.db -> clean

So the issue appears to be the eager Alembic import on the DB manager import path, and the lazy import in this PR removes the noise without changing the actual migration call sites.

[potiuk]

@hkc-8010 This PR has been converted to draft because it does not yet meet our Pull Request quality criteria.

Issues found:

• ❌ Failing CI: Special tests / Pendulum2 test: core / All-core:Pendulum2-Postgres:14:3.10:Core...Serialization, Special tests / Latest Boto test: providers / All-prov:LatestBoto-Postgres:14:3.10:-amazon,celer...standard, Special tests / Pendulum2 test: providers / All-prov:Pendulum2-Postgres:14:3.10:-amazon,celer...standard (+9 more). Please investigate and fix.

What to do next:

• Address the items above.
• Make sure static checks pass locally: prek run --from-ref main --stage pre-commit.
• Mark the PR as "Ready for review" when you are done.

Converting a PR to draft is not a rejection — it is an invitation to bring the PR up to the project's standards so that maintainer review time is spent productively. There is no rush — take your time and work at your own pace. We appreciate your contribution and are happy to wait for updates. If you have questions, feel free to ask on the Airflow Slack.

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