[python] avoid etag/match_condition clientName collision with multiple etag headers

[l0lawrence]

Problem

PR #10494 (which added support for custom etag wire names) broke operations that have more than one Azure.Core.eTag-typed header. The Azure Storage Blob copyFromUrl operation is a concrete example — it carries:

• standard If-Match / If-None-Match
• custom x-ms-source-if-match / x-ms-source-if-none-match

The emitter in http.ts stamps etagRole = "ifMatch" / "ifNoneMatch" onto every one of those headers. Then in preprocess/__init__.py, update_parameter blindly applies headers_convert(ETAG_MATCH_DATA) / ETAG_NONE_MATCH_DATA to every parameter with that role — overwriting clientName to "etag" and "match_condition" on both pairs. The operation ends up with two parameters named etag and two named match_condition.

The slot-picker in update_client already chose one ifMatch/ifNoneMatch slot per operation, but it did nothing to prevent the per-parameter rename on the others.

Fix

In preprocess/__init__.py update_client:

1. Collect all ifMatch and ifNoneMatch candidates per operation, not just the first.
2. New _pick_etag_slot helper prefers the standard If-Match / If-None-Match wire names over custom etag headers (matches pre-PR-10494 behaviour when both are present).
3. Strip etagRole from non-selected candidates so update_parameter leaves their natural clientName intact (e.g. source_if_match, source_if_none_match).

Result

For copyFromUrl:

• If-Match / If-None-Match → promoted to the etag / match_condition pair (unchanged behaviour vs. pre-PR-10494).
• x-ms-source-if-match / x-ms-source-if-none-match → keep their natural source_if_match / source_if_none_match client names.

No more clientName collisions.

Tests

Adds tests/unit/test_preprocess_etag.py with six tests:

• test_etag_role_preserved_when_only_standard_pair_present
• test_etag_role_preserved_when_only_custom_pair_present
• test_standard_etag_wins_over_custom_when_both_present (regression)
• test_first_custom_pair_chosen_when_multiple_custom_pairs_present
• test_synthetic_partner_still_works_with_only_one_custom_etag
• test_full_update_yaml_does_not_collide_client_names (end-to-end)

Verified the three multi-etag tests fail on upstream/main and pass with this change. All existing unit tests still pass.

Spec referenced

The original repro is copyFromUrl in specification/storage/Microsoft.BlobStorage/routes.tsp of Azure/azure-rest-api-specs, which composes SourceIfMatchParameter, SourceIfNoneMatchParameter, IfMatchParameter, and IfNoneMatchParameter (all Azure.Core.eTag).

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npm i https://pkg.pr.new/@typespec/http-client-python@10816

commit: f4c7af2

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-python

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@typespec/http-client-python - fix ✏️

Fix etag/match_condition clientName collision when an operation has more than one Azure.Core.eTag-typed header (e.g. Storage's copyFromUrl, which has both If-Match/If-None-Match and x-ms-source-if-match/x-ms-source-if-none-match). The standard If-Match/If-None-Match pair is now preferred for the etag/match_condition slot, and any additional etag-typed headers retain their natural client name (e.g. source_if_match).

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