[Amir-Golmoradi]

Fixes #9814

#7319 stopped adding Content-Length to multipart parts automatically, but Part.create still rejects it when supplied explicitly. Callers therefore have no way to include the header.

I removed that check. The writer already handles caller-supplied headers and counts their bytes toward the total multipart length. Parts without an explicit header are unchanged.

The outer length is still calculated from each body’s contentLength(). If a body returns -1, the multipart length remains unknown, regardless of the supplied header.

Tests cover both the factory and builder paths, the serialized output, the total byte count, and an unknown-length body with an explicit header. The existing rejection test now verifies that the builder preserves the header.

This patch allows explicit Content-Length headers for all multipart types. It does not include the FORM-specific restriction or value validation proposed in the issue discussion.

The multipart tests, spotlessCheck, and the full check pass.

[Amir-Golmoradi]

@yschimke, could you take a look when you have a chance? You reviewed #7319, which this follows up on.