[abhijeets25012-tech]

Adds Python task handler support for YAML Dag run tasks.

• Introduces the public task_handler(dag_id=..., task_id=...) decorator and a registry for handler functions.
• Adds PythonCoordinator to load and resolve handlers from the configured bundle.
• Dispatches stub tasks to their registered handlers at runtime.
• Validates run task arguments against handler signatures when possible.
• Documents handler setup and usage.
  Closes Implement Python task_handler decorator for YAML Dag ‘run’ tasks #75273
  Verification
• Ruff format and Ruff check passed for changed Python files.
• git diff --check passed.
• Tests were not run. breeze verify could not run because the installed uv version did not meet the repository requirement.

[kaxil]

@abhijeets25012-tech Closing this. #75273 is assigned to a maintainer who has already linked #75405 to it, so commenting "I will do it" and opening a PR 25 minutes later is not how this works.

Also, the PR says no tests were run, and for a change like this unit tests alone wouldn't be enough anyway. We need real evidence it works end to end: screenshots, a demo video or a gif of a YAML Dag actually running its handler.

Please check an issue is unassigned before you pick it up, and maybe start with the good first issue ones.