Fix: parse mypy diagnostics from stderr in non_interactive mode

[cnaples79]

Summary

When mypy is run with non_interactive = True, it can emit diagnostics to stderr instead of stdout. The extension currently parses only stdout, resulting in no diagnostics being shown even though errors are present.

Changes

• bundled/tool/lsp_server.py: If either stdout or stderr has content, aggregate both and parse the combined output. Continue logging stdout as before; stderr is already logged separately.

Rationale

• Matches observed behavior in issue Plugin silently fails when mypy non_interactive option is set #364 where diagnostics appear in logs but are not processed.
• Maintains existing logging behavior while ensuring diagnostics are not dropped.

Test Plan

• Repro with a setup.cfg or pyproject.toml containing:

  [mypy]
  install_types = True
  non_interactive = True
  

• Introduce obvious type errors and confirm diagnostics now appear in the editor.

Fixes #364