fix: Remove default content-length header from MultipartStreamBuilder class

[chris-lee-lb]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	N/A
Documentation	N/A
License	MIT

What's in this PR?

Remove default Content-Length header from MultipartStreamBuilder class.

Why?

Content-Length in multipart bodies is not necessary. Some legacies system can't manage the "Content-Length" header into "multipart/form-data" boundary and cut off the request.

Currently I had face this problem with Azure OpenAI upload files API.

For reference see the RFC 7578 in section 4.8 which specify:

4.8. Other "Content-" Header Fields

The multipart/form-data media type does not support any MIME header
fields in parts other than Content-Type, Content-Disposition, and (in
limited circumstances) Content-Transfer-Encoding. Other header
fields MUST NOT be included and MUST be ignored.

Older system are not compliant because not apply the MUST be ignore policy but on the other side this code non apply the MUST NOT be included policy. So the Azure OpenAI API server is designed to be RFC compliant.

Example Usage

N/A

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

[chris-lee-lb]

Some another library references : guzzle/psr7#581

[dbu]

thanks for the RFC reference. yeah makes sense.

also boo microsoft for not being robust.

[dbu]

sorry, will do the weird styleci thing. not related to your changes.

we should use the phpstan array format for this parameter actually... but out of scope for this pr