Test: Add comprehensive tests for rcParams support

Manual test notebook (tests/manual_test_rcparams_save.ipynb):
- Tests all savefig.* rcParams (format, transparent, facecolor, dpi)
- Tests multiple formats: PNG, PDF, SVG, JPEG
- Includes verification checklist
- Documents expected behavior for each test case

Python unit tests (tests/test_download.py):
- Test _send_save_buffer respects savefig.format (PNG, PDF, SVG)
- Test download() method calls _send_save_buffer
- Test Toolbar.save_figure() calls _send_save_buffer
- Test respects savefig.dpi and savefig.transparent rcParams
- Test warns on unsupported format (e.g., webp)
- Add 'tests' to pytest testpaths in pyproject.toml

Addresses issues #138, #234, #339

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ianhi

pyproject.toml

@@ -116,6 +116,7 @@ source = "code"
 
 [tool.pytest.ini_options]
 testpaths = [
+    "tests",
     "docs/examples",
 ]
 norecursedirs = [