test(api): lock the filtered models response against a stale content-length

[HouMinXi]

Summary

• The provider models route rewrites the catalog body down to one provider. The upstream content-length belongs to the full catalog, and forwarding it makes the client wait for bytes that never arrive. That was fixed earlier, but nothing kept it fixed.
• This adds a regression test. It plants a catalog response whose content-length is the full catalog and asserts the filtered response drops it, while a body passed through unchanged keeps its own length.
• The route exposes the catalog loader as a field so the test can plant that response. The field points at the real loader in production.

Related Issues

• Related to fix(api): GET /api/v1/providers/{provider}/models copies Content-Length of the full /v1/models catalog — response never completes, Playground model list stuck on "loading" #15212
• Related to fix(api): /api/v1/providers/{provider}/models forwards the unified catalog's content-length onto a re-serialized filtered body — response never completes (playground model dropdown stuck loading) #14092

Validation

• Change type: other (regression test)
• Focused tests: node --import tsx/esm --test tests/unit/provider-models-content-length-15212.test.ts (2 pass)
• eslint on the changed files, with the repo suppression file
• Reconciled with release/v3.8.52 @ 23a1148486
• New test covers the change
• Defect injection: replacing stripStaleEncodingHeaders with a raw header copy makes the stale-length assertion fail, and the change was restored

Tests Added Or Updated

• tests/unit/provider-models-content-length-15212.test.ts (added)

Coverage Notes

• One case filters a catalog and asserts the stale length and encoding are gone. A second case passes a non-JSON body through and asserts its length stays.

Reviewer Notes

• The stripping itself landed in fix(api): drop stale catalog content-length when re-serializing model responses (#14092) #14120. This change does not touch it. The only production edit is the unifiedModels field, which exists so the test can reach the handler with a planted response.