fix: restore baseline relay test behavior

[lyz52789]

What changed

• restore Claude relay handling for inline PDF and plain-text file content while skipping unsupported file types
• preserve a caller-provided StreamStatus instead of replacing it at stream startup

Why

The existing baseline tests exposed two regressions:

• generalized media conversion lost the filename-derived MIME type and treated file content as an image
• StreamScannerHandler unconditionally replaced a pre-initialized status, discarding errors already recorded by its caller

Scope

This PR is separate from #14 so the Responses arguments compatibility patch remains independently reviewable and deployable.

Validation

The failures were reproduced on origin/main before applying these changes. After the fix, the full Go suite passes from a clean worktree prepared the same way as the Docker build:

mkdir -p web/dist
cp web/index.html web/dist/index.html
go test ./... -count=1

[cloudflare-workers-and-pages]

Deploying new-api with    Cloudflare Pages

Latest commit:	ea58e9a
Status:	✅  Deploy successful!
Preview URL:	https://238eb3d7.new-api-d57.pages.dev
Branch Preview URL:	https://agent-fix-baseline-go-tests.new-api-d57.pages.dev

View logs