ci: Python emitter regen-diff check (PoC)

[l0lawrence]

What

Proof-of-concept CI check that detects when a change to the Python emitter (packages/http-client-python) alters its generated test code.

On a PR touching packages/http-client-python/**, the workflow:

1. Regenerates the emitter's generated test code twice — once with the emitter at the PR head, once with the emitter at a pinned baseline commit stored in packages/http-client-python/eng/regen-diff-baseline.txt.
2. Diffs the two generated trees.
3. Fails CI if there is any diff.
4. Uploads a colorized HTML view of the diff as a workflow artifact (python-regen-diff-html).
5. Posts/updates a PR comment (via the secure workflow_run pattern) linking to the artifact.

If a diff is expected, the author bumps the baseline SHA to accept the new output.

Files

• packages/http-client-python/eng/regen-diff-baseline.txt — pinned baseline commit SHA.
• packages/http-client-python/eng/scripts/ci/regen-diff-html.ts — dependency-free reporter (git diff --no-index → HTML + comment payload + result.json).
• .github/workflows/python-emitter-regen-diff.yml — pull_request: generate (head/base matrix) → diff → fail-on-diff.
• .github/workflows/python-emitter-regen-diff-comment.yml — workflow_run: posts the PR comment (works for fork PRs).

Notes / caveats

• Cost: regenerates twice per PR (full Python setup each time). A follow-up could cache the base regen keyed by the baseline SHA to halve the work.
• Both regens fetch the same production baseline seed, so it cancels out in the diff; current-date values also cancel since both run in the same CI run (hence no cross-day caching).
• Proof-of-concept — Python only. A sibling PR ports the same setup to Azure/typespec-azure. Intended to be extended to the csharp/java/js emitters next.

[github-actions]

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The following packages have changes but are not documented.

• ❌@typespec/http-client-python

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