Support IFormFile binding on complex [FromForm] models

[jeremydmiller]

Summary

• Fixes Improve support for [FromForm] so that file uploads can be bound to complex models directly #1705: IFormFile and IFormFileCollection properties on complex models annotated with [FromForm] are now automatically bound from multipart form data
• Previously, only direct method parameters of type IFormFile/IFormFileCollection were supported via FromFileStrategy. The FormBindingFrame that handles complex model property binding called TryFindOrCreateFormValue() for each property, which had no handling for file types — silently skipping them
• Added ReadFormFileByName helper to HttpHandler that reads a form file by field name via IFormFileCollection.GetFile()
• Added FormFilePropertyFrame and FormFileCollectionPropertyFrame code generation frames for property-level file binding
• Updated FormBindingFrame to detect IFormFile/IFormFileCollection in both constructor parameters and settable properties
• Also accepts multipart/form-data content type on all form data endpoints alongside application/x-www-form-urlencoded

Test plan

• New test: bind_single_file_on_complex_model — verifies IFormFile property is bound with correct filename and length
• New test: bind_file_collection_on_complex_model — verifies IFormFileCollection property is bound with correct file count
• New test: bind_file_on_complex_model_when_no_file_sent — verifies IFormFile property is null when no file is sent
• All existing from_form_binding tests pass (11 passed, 2 skipped due to pre-existing Alba limitation)
• Full HTTP test suite: 463 passed, 4 pre-existing failures unrelated to this change

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