[codex] Improve Content-Disposition parameter parsing

[chrislearn]

Summary

• parse Content-Disposition parameter names case-insensitively
• decode percent-encoded unquoted values and support filename*
• add end-to-end coverage for the compatibility cases

Why

The old parser only matched lowercase name/filename and ignored RFC 5987-style filename* values. That caused valid or common client uploads to lose the field name or filename.

Impact

Multipart parsing becomes more compatible with real-world clients without changing the public API.

Validation

• cargo fmt --all
• cargo test --all --all-features
• cargo clippy --tests --examples --bins -- -D warnings

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