Fix Python post-processing scope to preserve user-managed files

[Libba Lawrence (l0lawrence)]

The Python emitter's post-generation steps currently scan the output directory, formatting and adding pylint suppression comments to pre-existing tests, samples, and other user-managed Python files. This change limits those steps to files written during the current generation.

Approach

• Record successful writes at the serializer's shared writer layer using normalized absolute paths. Reset tracking on each generation so empty runs and reruns cannot fall back to processing stale files.
• Use one Python pipeline for native and Pyodide generation, explicitly passing recorded paths to Black formatting and suppression insertion. Batch workers return their paths to the parent for post-processing.
• Preserve regenerated SDK files, packaging files, generated tests/samples, namespace layouts, and paths outside the immediate generation directory. No public emitter option is added.

This post-generation step does not invoke pylint; it inserts suppression comments where necessary. The standalone pygen.black command now requires explicit filenames. Suppression insertion also preserves stable line endings when extending an existing header.

Validation

• 63 focused Python regression and adjacent unit tests passed, including byte-for-byte preservation of BOM/CRLF user files, regeneration, empty file sets, threshold boundaries, generated test/sample gating, namespace layouts, the native entry point, and real batch multiprocessing.
• All 26 emitter tests passed, including native and Node/browser Pyodide routing regressions. Pyodide runtime objects are mocked; the shared Python pipeline is exercised with CPython, not live WASM.
• TypeScript compilation and generator wheel build passed. Package formatting and emitter lint passed; targeted Python lint, type checks, and whitespace checks passed.

Includes a fix changelog entry and documentation describing the post-processing scope.

Fixes: #11424

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-python@12114

commit: ea6de3f

[azure-sdk-automation]

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[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-python

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@typespec/http-client-python - fix ✏️

Limit Black formatting and pylint suppression insertion to Python files written during the current generation, leaving pre-existing user-managed tests, samples, and other Python files unchanged.

[github-actions]

Python emitter diff

Baseline gh:be4c4b7f5646d7eea4affd972d5f69de6c21fdd1 vs this PR.

Diff summary: 1 file(s), +1 / -1

Rendered diff: inline on the run summary, or the emitter-diff-html artifact.

Informational check (eng/emitter-diff); does not block the PR.