Agents v2 (branch-to-branch): port workarounds for AOAI (FDP) openai files quirks

[trrwilson]

When (and only when) using fine-tune as the purpose for uploading files, the OpenAI "Create File" operation behaves differently in AOAI/FDP relative to OpenAI:

• A Content-Type header is required on the multipart/form-data part for file
• A success code of 201 is used instead of 200
• A different set of status values (including "pending" and "processed") is used

The Azure OpenAI library, by virtue of its [InternalsVisibleTo] relationship with the OpenAI library, implements a full replacement of some of the logic to account for this. We don't want to and can't have this kind of relationship going forward, so we need to achieve the same workarounds in a different way.

With this change, we use a pipeline policy to:

• Insert the requisite Content-Type header in create file requests
• Replace the default HTTP 200 classifier with a 200/201 classifier
• Move parallel "status" values into a separate additional property (and align nominal value with OpenAI)

Mirroring Azure OpenAI, an extension method is also provided for OpenAIFile that can retrieve the wire value initially received.

Net effect: an OpenAIClient retrieved via an AgentsClient can now successfully use create/get file operations.