tests(run): Add comprehensive tests for output streaming and flushing behavior

- Add TestFlushingBehavior class with tests for:
  - Timing verification of immediate flushing
  - Unbuffered subprocess output handling
  - Carriage return overwrites for progress bars
  - ANSI escape sequence preservation
  - Git-style progress simulation
  - Mixed line endings (\n, \r, \r\n)
  - Large output streaming without blocking
  - stderr vs stdout stream separation

- Add TestRealWorldScenarios class covering:
  - npm-style progress with Unicode spinners
  - Long-running processes with periodic updates
  - Multi-line progress bars (Docker-style)
  - Binary/Unicode output handling
  - Interleaved stdout/stderr timing

- Add TestEdgeCases class for:
  - Empty output handling
  - stdout-only processes
  - Very long lines exceeding buffer size
  - Rapid output bursts
  - Callback exception handling

- Enhance existing tests with better type annotations
- Fix linting issues (imports, line length, docstrings)
- Follow libvcs testing conventions and patterns

These tests verify the current behavior of the run() function's
real-time output streaming via callbacks, including the 128-byte
chunking behavior and proper handling of control characters.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: tony