Fix duplicate plugin names by surfacing import errors

[LarytheLord]

Summary

Fixes #55140.

When two plugins share the same AirflowPlugin.name, Airflow currently logs a warning and silently skips the duplicate. This can hide the real problem from users (especially when only one plugin appears in UI/API).

This change keeps the current non-blocking behavior (subsequent plugins still load), but also records duplicate plugin-name conflicts as import errors so the issue is visible and actionable.

Changes

• In airflow.plugins_manager._get_plugins():
  • track loaded plugin names with their source
  • on duplicate name:
    • keep warning log
    • add an explicit entry to import_errors
• Update unit test coverage in test_plugins_manager.py:
  • test_duplicate_plugin_name_is_reported_and_does_not_prevent_loading_subsequent_plugins
  • verifies duplicate is reported in import_errors while unique plugins still load

Why this approach

• Surfaces the conflict as a concrete error instead of only a warning.
• Preserves existing resilience: one bad/duplicate plugin does not block other plugins.

[boring-cyborg]

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[LarytheLord]

Follow-up validation on this branch:

• uvx ruff check airflow-core/src/airflow/plugins_manager.py airflow-core/tests/unit/plugins/test_plugins_manager.py
• uvx ruff format --check airflow-core/src/airflow/plugins_manager.py airflow-core/tests/unit/plugins/test_plugins_manager.py

Both pass.

I also attempted to run the targeted pytest case for test_plugins_manager.py -k duplicate_plugin_name, but this local environment started bootstrapping the full monorepo dependency set and did not complete in a reasonable time window. CI on the PR should run the full project test matrix.

[LarytheLord]

Rebased on latest main to satisfy up-to-date checks. No functional changes beyond the existing fix + test.