Adds validation to schedule_task() to catch schema mismatches early,
before tasks are scheduled to the task broker. This prevents task
failures when model changes are deployed to task brokers before
Sentry workers are updated.

Changes:
- Validate transformed event payload using Pydantic models before
  scheduling the Celery task
- Use appropriate model (SeerCodeReviewTaskRequestForPrClosed or
  SeerCodeReviewTaskRequestForPrReview) based on request_type
- Log validation failures with detailed context
- Record INVALID_PAYLOAD metric when validation fails
- Add tests to verify validation happens before task scheduling

Fixes: CW-837

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

Removes validation logic from process_github_webhook_event task since
payload is now validated before scheduling in schedule_task().

This prevents double validation and ensures the validation happens
at the right layer - before tasks are queued to the broker rather
than during task execution.

Changes:
- Remove Pydantic validation from process_github_webhook_event
- Move enum key conversion to schedule_task after validation
- Update docstring to clarify payload is pre-validated
- Simplify task execution to just forward payload to Seer

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

- Integrate tags parameter added in master
- Keep pre-scheduling validation logic
- Remove duplicate validation from task execution
- Revert unrelated changes to pull_request.py and issue_comment.py

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

Remove unused 'e' variable from exception handler to fix linting.

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

Remove tests that expected validation errors during task execution,
since validation now happens before scheduling. Validation at the
scheduling layer is already covered by tests in test_pull_request.py.

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

Address Bugbot concerns:
1. Catch only pydantic.ValidationError instead of all exceptions
   to avoid hiding unexpected failures
2. Add datetime serialization in convert_enum_keys_to_strings()
   to ensure Celery can serialize all payload values

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>

orjson.dumps() in make_seer_request() already handles datetime
serialization automatically. Converting datetime to strings early
breaks tests that expect datetime objects in the payload.

Co-authored-by: Armen Zambrano G. <armenzg@users.noreply.github.com>