Experiment: defer heavy imports in logging handlers

[zzstoatzz]

Summary

This PR is a focused experiment to reduce common CLI startup overhead by deferring heavy imports in prefect.logging.handlers until they are actually needed.

It intentionally does not change plugin initialization semantics or prefect.context behavior.

What changed

• Replaced eager module-level imports in src/prefect/logging/handlers.py with cached local loader helpers for:
  • prefect.context
  • prefect.client.orchestration.get_client
  • prefect.client.schemas.actions.LogCreate
  • logging settings constants from prefect.settings
  • from_timestamp and MissingContextError
• Updated handler codepaths to use those helpers lazily.

Why

This is the narrowest low-risk candidate from recent CLI startup investigations. If it helps, we can keep it; if it does not, this still gives us concrete data for future import-graph work.

Local validation

• uv run pytest tests/test_logging.py -q
• uv run pytest tests/cli/test_version.py tests/cli/test_config.py -q
• uv run ruff check src/prefect/logging/handlers.py

Local bench note

Local cli-bench results were noisy and not consistently positive on my machine, so this is being sent to CI as an empirical check rather than a claimed win.

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing codex/cli-startup-logging-handlers (a99da08) with main (f8874f4)}