refactor!: complete deprecation and cleanup of multimodal blob APIs

[shuoweil]

This PR completes the deprecation and cleanup of the public multimodal blob APIs in BigFrames. These APIs were not intended for general public use and are being internalized or removed to clean up the public API surface.

Key Changes:

• API Internalization:

  • Renamed BlobAccessor to _BlobAccessor and internalized the .blob accessor on Series and DataFrame to ._blob.
  • Internalized Series.str.to_blob to Series.str._to_blob.

• Deprecations:
  Deprecated Session.from_glob_path. It will now emit a warning suggesting users use read_gbq with a reference column instead.

• Cleanup:

  • Removed bigframes/blob/_functions.py and associated large system tests.
  • Removed small system tests for blob operations in tests/system/small/blob/ (since they relied on the public .blob accessor).
  • Removed BlobAccessor from the API documentation reference in toc.yml.
  • Annotated cells in the Kaggle notebook vector-search-with-bigframes-over-national-jukebox.ipynb to note the deprecation of str.to_blob and removal of .audio_transcribe.

Fixes #<478952827> 🦕

[gemini-code-assist]

Code Review

This pull request transitions the blob accessor to an internal implementation (_blob) and removes or deprecates several public blob-related functions, including image and PDF processing utilities, in favor of bigframes.bigquery.obj functions. It also includes reformatting of various overloaded methods across the codebase. Feedback was provided to improve an error message in strings.py by making it dynamically reflect the method name to avoid confusion between public and internal methods.