Inject generated boundary into multipart/form-data Content-Type header

[mcmxcdev]

Fixes #885

Problem

When a user supplies a multipart/form-data Content-Type header without an explicit boundary parameter, httpx generated a boundary for the request body but left it out of the header. The body contained correct --<boundary> delimiters while the header read e.g. multipart/form-data with no boundary. Servers then rejected the request (400, missing form params).

Fix

When the user-supplied content-type is multipart/form-data with no boundary, the generated boundary is now appended to the header so it matches the boundary used in the body.

• is_multipart_form_data_content_type() — case-insensitive media-type check accepting str/bytes/None.
• append_boundary_to_content_type() — appends ; boundary=..., normalizing trailing ;/whitespace.
• Request.__init__ injects the generated boundary into the header when none was supplied.

Tests

Extended test_multipart_header_without_boundary to assert the header now carries a 32-char hex boundary matching the body delimiters, plus unit tests for the two new helpers. All 50 tests in test_multipart.py pass.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing mcmxcdev:fix/multipart-boundary-header-885 (100fb71) with main (82b9e2d)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}