Sync PR head with microsoft/typespec main before regeneration

[msyyc]

Summary

When the TypeSpec Python regeneration workflow runs against a microsoft/typespec pull request URL, it now merges microsoft/typespec main into the checked-out PR head before regenerating. This ensures the regenerated diff reflects only the PR's changes on top of current main, rather than including changes the PR is simply behind on.

Changes

• Added a Sync PR head with microsoft/typespec main step that runs only for PR-sourced runs (typespec_pr_number != '').
• The step adds microsoft/typespec as an upstream remote (idempotently via set-url/add), fetches its main, and merges it into the PR head.
• If the PR conflicts with main, the merge is aborted and the workflow fails with a clear, actionable message instructing the user to update the PR branch and resolve conflicts before re-running.

Notes

• The existing _typespec checkout already uses fetch-depth: 0, so the full history is available for a proper merge base.
• No behavior change for main-sourced runs (push/schedule/main input).

Test

• https://github.com/Azure/azure-sdk-for-python/actions/runs/28433736771/job/84254467656
• https://github.com/Azure/azure-sdk-for-python/actions/runs/28433875046/job/84254938607

[Copilot]

Pull request overview

This PR updates the TypeSpec Python Regenerate Tests GitHub Actions workflow so that, when triggered against a microsoft/typespec pull request URL, it merges microsoft/typespec main into the checked-out PR head before regenerating tests. This ensures the regenerated diff reflects only the PR's own changes relative to current main, rather than also surfacing unrelated emitter changes the PR is merely behind on.

Changes:

• Added a Sync PR head with microsoft/typespec main step gated to PR-sourced runs (typespec_pr_number != '').
• The step idempotently configures an upstream remote pointing at microsoft/typespec, fetches its main, and merges it into the detached PR head checkout.
• On merge conflict, it aborts the merge and fails the run with an actionable error message; on success it emits a notice with the resulting short SHA.