[krishan711]

Description

json_route and streaming_json_route (so also route) now accept multipart/form-data bodies, so API routes can receive file uploads.

• Body parsing moves to core.api.request_body.read_request_body, which both routes use. JSON bodies work as before.
• Multipart form fields are used like JSON body fields, so pydantic still validates and coerces them (e.g. "41" to 41). Uploaded files are passed as the new core.api.uploaded_file.UploadedFile (filename, contentType, content), so request models declare file: UploadedFile.
• A form field given more than once raises BadRequestException (400).
• The core-api extra now installs python-multipart, which starlette's form parsing needs.

The local full test run has 37 failures in tests/caching and tests/util/test_file_util.py. The same 37 fail on main locally. tests/api passes, including the 2 new multipart tests.

Checklist:

• I have updated the CHANGELOG with a summary of my changes