Add request.form data migration support

[zmievsa]

Expose a mutable form property on RequestInfo so users can read and modify multipart/form-encoded fields (strings and file uploads) inside convert_request_to_next_version_for handlers. The property returns None for non-form requests, matching the existing pattern for missing bodies. Mutated form data is reconstructed into a new FormData before being passed to FastAPI's dependency solver.

[codecov]

Codecov Report

❌ Patch coverage is 96.84211% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.84%. Comparing base (62e15d1) to head (73ef8e7).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_data_migrations.py	96.38%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
- Coverage   99.89%   99.84%   -0.06%     
==========================================
  Files          72       72              
  Lines        5553     5645      +92     
  Branches      356      358       +2     
==========================================
+ Hits         5547     5636      +89     
- Misses          6        9       +3     

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[zmievsa-semasoftware]

Code review

Found 2 issues:

1. Multi-value form fields are silently truncated. dict(body.multi_items()) discards all but the last value for repeated keys (e.g. tag=a&tag=b → {"tag": "b"}), and the reconstruction in versions.py compounds this with .items() on the lossy dict. Any endpoint using List[str] = Form(...) or repeated form fields will silently receive truncated data even when no migration is defined.

cadwyn/cadwyn/structure/data.py

Lines 27 to 30 in e9e20b4

	if isinstance(body, FormData):
	self._form: Union[dict[str, Union[UploadFile, str]], None] = dict(body.multi_items())
	else:
	self._form = None

cadwyn/cadwyn/structure/versions.py

Lines 434 to 437 in e9e20b4

	CURRENT_DEPENDENCY_SOLVER_VAR.set("cadwyn")
	body_for_solving = (
	FormData(list(request_info._form.items())) if request_info._form is not None else request_info.body
	)

2. _get_body has # pragma: no cover # This is from FastAPI on its definition, but the new form migration tests now call this function. The pragma excludes the entire function from coverage tracking, which is now inaccurate. Removing it may require adding coverage for any remaining uncovered branches inside the function.

cadwyn/cadwyn/structure/versions.py

Lines 740 to 744 in e9e20b4

	async def _get_body(
	request: FastapiRequest, body_field: Union[ModelField, None], exit_stack: AsyncExitStack
	): # pragma: no cover # This is from FastAPI
	is_body_form = body_field and isinstance(body_field.field_info, params.Form)
	try:

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