Add x-cog-accept MIME type annotation for Path/File input fields
Add an 'accept' parameter to Input() that specifies allowed MIME types
or file extensions for Path/File inputs. The Go static schema generator
extracts this and emits it as 'x-cog-accept' in the OpenAPI schema,
giving schema consumers (UIs, validators, API clients) visibility into
what file types an input expects.
Usage: Input(accept="image/*"), Input(accept="audio/wav,audio/mp3"),
or Input(accept=".safetensors,.bin"). Using accept on non-Path/File
types is a hard build error.
