fix(filesystem): reject non-media read_media_file outputs

[go165]

Summary

• remove the invalid blob content type from read_media_file output schema
• return a tool error for non-image/audio files instead of emitting a non-spec content block
• add integration coverage for image success and non-media rejection

Fixes #4029

Validation

• npm exec --workspace src/filesystem -- tsc --noEmit
• npm run --workspace src/filesystem build
• npm exec --workspace src/filesystem -- vitest run __tests__/structured-content.test.ts --coverage=false
• git diff --check

Note: validation used temporary symlinks to the existing local node_modules directories from the main checkout; no new install was run.