fix(deepagents): default unknown file extensions to text/plain (#656)

### Summary

`read_file` couldn't read many common text files and with Anthropic it
crashes the whole run. `getMimeType()` mapped any unrecognized extension
to `application/octet-stream`, which `isTextMimeType()` treats as
binary, so the file got base64-encoded into a document block.

This defaults unknown extensions to `text/plain` instead. Known binaries
(images, audio, video, PDF/PPT) stay explicitly mapped, so image and PDF
handling is unchanged. Files like `.properties`, `.scss`, `.tf`,
`.lock`, and extension-less files like `Dockerfile` now read as text.

  ### Tests
  
Flipped the unknown-extension unit test to expect `text/plain`.