refactor: break up ChecksNotifier.notify into focused methods [2/4]

[thomasrockhu-codecov]

Summary

Extracts ChecksNotifier.notify() (176 lines) into focused methods under 50 lines each. Pure refactoring, no behavior changes. Removes unused nullcontext and get_commit_url imports.

Method	Responsibility
notify()	Orchestrates: PR validation → preconditions → status check → build → send
_validate_pull_request()	Validates PR exists, is in provider, is open
_check_preexisting_status()	Checks if commit status already exists
_build_payload_with_behavior()	Overrides parent with checks-specific output logic

Inherits _check_notify_preconditions() and _set_payload_url() from StatusNotifier (PR 1/4).

Test plan

• test_checks.py: 57/57 passed
• ruff check + ruff format: passing

Stack: This is PR 2/4. Depends on PR 1/4 (#694).

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[sentry]

Codecov Report

❌ Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.35%. Comparing base (4935682) to head (814ca05).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ker/services/notification/notifiers/checks/base.py	95.23%	2 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##           refactor/status-notifier-breakup     #695   +/-   ##
=================================================================
  Coverage                             92.34%   92.35%           
=================================================================
  Files                                  1302     1302           
  Lines                                 47818    47829   +11     
  Branches                               1619     1619           
=================================================================
+ Hits                                  44159    44171   +12     
+ Misses                                 3350     3349    -1     
  Partials                                309      309           

Flag	Coverage Δ
workerintegration	58.57% <7.14%> (-0.03%)	⬇️
workerunit	90.39% <95.23%> (+0.01%)	⬆️

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Codecov Report

❌ Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ker/services/notification/notifiers/checks/base.py	95.23%	2 Missing ⚠️

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