Add workflow to sync rush.json with Dependabot updates

[nev21]

This workflow automatically syncs the rush.json version with changes made by Dependabot to package.json and related files. It detects changes, updates the rush.json version, runs rush update, and commits the changes if any are detected.

[Copilot]

Pull request overview

This PR adds a GitHub Actions workflow to automatically synchronize the rush.json configuration file when Dependabot updates the @microsoft/rush dependency in package.json files. The workflow detects Rush version changes, updates the rush.json file accordingly, runs rush update to regenerate lockfiles, and commits the changes back to the pull request.

Key changes:

• New automated workflow triggered on Dependabot pull requests that modify package.json, rush.json, or common-versions.json files
• Two-job workflow: detection job identifies if @microsoft/rush was updated, and update job syncs rush.json and regenerates lockfiles
• Automated commit and push mechanism to apply changes directly to Dependabot PRs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.89%. Comparing base (746ce64) to head (8d73695).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #483   +/-   ##
=======================================
  Coverage   98.89%   98.89%           
=======================================
  Files         108      108           
  Lines        2981     2981           
  Branches      621      622    +1     
=======================================
  Hits         2948     2948           
  Misses         33       33           

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