Fix KeyError when log event is missing 'level' key in TriggerRunner

[Srujan-rai]

Triggerer subprocess log reader called event.pop("level") without a
default, raising KeyError when a structlog event omits the "level" field.
Changed to event.pop("level", "warning") so missing-level events are
logged at WARNING instead of crashing the log processing loop.

Added tests covering missing level, unknown level, and valid level paths.

[boring-cyborg]

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[potiuk]

@Srujan-rai This PR has been converted to draft because the Core...Serialization test suite is failing across every database backend (Postgres, MySQL, SQLite, and the LowestDeps matrix), which indicates the change has broken serialization logic — see our Pull Request quality criteria.

Issues found:

• ❌ Core serialization tests failing on all DBs:

  • Postgres tests: core / DB-core:Postgres:14:3.10:Core...Serialization — failure
  • MySQL tests: core / DB-core:MySQL:8.0:3.10:Core...Serialization — failure
  • Sqlite tests: core / DB-core:Sqlite:3.10:Core...Serialization — failure
  • Low dep tests:core / All-core:LowestDeps:14:3.10:Core...Serialization — failure

  The failure being symmetric across all DB backends means it is not flaky CI — the test code itself is failing against the PR's change. Likely the new KeyError guard interacts with how triggerer log records are serialized. Reproduce locally with:

  breeze testing core-tests --test-types Core --backend sqlite -- -k serialization
  

  See the testing docs.

What to do next:

• Run the serialization tests locally, fix the regression, push.
• Mark the PR as "Ready for review" when CI is green.

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. No rush — take your time. If you have questions, feel free to ask on the Airflow Slack.

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