fix(mcp): align OpenAI file parameter schema

[uvwt]

Summary

• expose file_publish.file as an open object instead of string/binary
• keep path as the normal local-path entrypoint
• update OpenAI file metadata regression test

Why

OpenAI now requires parameters referenced by openai/fileParams to resolve to an object or an array of objects.

Validation

• go test ./internal/tool/media ./internal/mcp ./internal/app
• go test ./...
• git diff --check