Add x-cog-accept MIME type annotation for Path/File input fields

[markphelps]

Summary

• Adds an accept parameter to Input() for declaring allowed MIME types / file extensions on Path and File inputs (e.g. Input(accept="image/*"), Input(accept="audio/wav,audio/mp3"), Input(accept=".safetensors,.bin"))
• The Go static schema generator extracts this and emits x-cog-accept in the OpenAPI schema, giving schema consumers (UIs, validators, API clients) visibility into what file types an input expects
• Using accept on non-Path/File types (e.g. str, int) is a hard build error

Example

class Predictor(BasePredictor):
    def predict(
        self,
        image: Path = Input(description="Input image", accept="image/*"),
        audio: Path = Input(description="Audio clip", accept="audio/wav,audio/mp3"),
    ) -> Path:
        ...

Produces:

{
  "image": {
    "type": "string",
    "format": "uri",
    "x-cog-accept": "image/*"
  },
  "audio": {
    "type": "string",
    "format": "uri",
    "x-cog-accept": "audio/wav,audio/mp3"
  }
}