Add basic support for UploadedFile UserContent

[tarruda]

@DouweM here's some initial support for #2574

The UploadedFile user content is simply wrapping an opaque reference to some return value of provider-specific file upload API, which is then validated in the corresponding model _map_user_prompt.

I've only added support for Google and OpenAI, but I believe the API should be flexible enough to add support for other providers. I started working on Anthropic, but decided to leave it out for now as the official SDK doesn't support this feature yet, and I was having trouble referencing it using the SDK data objects.

I've opted to not implement a Provider.upload_file abstraction, as the options can be different across providers and I would need to get more familiar with pydantic-ai before feeling confident enough to design a proper abstraction (Can follow up with another PR later!)

One caveat with the tests: The VCR framework apparently doesn't support requests containing binary content, so I had to turn off for uploading files. This is how I proceeded to add the tests:

• Wrote code to upload the file (a new smiley pdf which I've added to tests/assets/smiley.pdf) and turned vcr off.
• Ran the test with a print statement to show the return value
• Commented the code to upload the file (left it as reference for later)
• Re-ran the tests with recording on, and with a literal provider-specific file object with the same id I had previously uploaded.

Since this is just a recording and we are only verifying that we can reference an uploaded file, it probably doesn't matter much that we are not actually running the upload request for now. This can be changed later when VCR is fixed to support this type of request.

Close #2574

[tarruda]

As shown in the google file upload example:

result = client.models.generate_content(
    model="gemini-2.0-flash",
    contents=[
        myfile,
        "\n\n",
        "Can you tell me about the instruments in this photo?",
    ],
)

The file is actually the whole content, instead of being a "message part". That's why I'm appending to the contents array directly here. Reference: https://ai.google.dev/api/files

[tarruda]

@DouweM CI still failing on code coverage. I will fix it, but first I'd love some feedback on the API . LMK if you agree with the choices or if I should make some adjustments!