Add merge commit message option to repo_update_pull_request

[simontherry]

Summary

• Add an optional mergeCommitMessage parameter to repo_update_pull_request.
• Pass the value through to completionOptions.mergeCommitMessage when autocomplete completion options are built.
• Add Jest coverage for the autocomplete path.

Why

Azure DevOps supports customizing the final merge or squash commit message through pull request completion options, but the MCP tool did not expose that field.

Fixes #1209

Validation

• npm test -- --runTestsByPath test/src/tools/repositories.test.ts
• npm run validate-tools

[simontherry]

@microsoft-github-policy-service agree

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@feac5c9). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1217   +/-   ##
=======================================
  Coverage        ?   90.85%           
=======================================
  Files           ?       19           
  Lines           ?     2175           
  Branches        ?      514           
=======================================
  Hits            ?     1976           
  Misses          ?      102           
  Partials        ?       97           

Flag	Coverage Δ
unittests	90.85% <100.00%> (?)

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