fix: trigger CI for backend update PRs

[MilesCranmerBot]

Backend-update PRs are created by a workflow using GITHUB_TOKEN, so the usual push/pull_request-triggered workflows don't run (GitHub blocks workflow-on-workflow recursion).

This PR:

• Adds workflow_dispatch to Linux/macOS/Windows CI workflows.
• Updates update_backend.yml to dispatch those CI workflows whenever it creates/updates a backend-update PR.

This keeps the automated backend bump PRs getting CI without requiring a PAT.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (95ffa50) to head (7002b2f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1158   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files          21       21           
  Lines        1687     1687           
=======================================
  Hits         1590     1590           
  Misses         97       97           

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