Add automated test suite and CI workflow

[gnodet]

Summary

The project currently has zero automated tests — no test files, no pytest configuration, no CI workflow for tests. The only CI job (docs.yml) deploys MkDocs. For a "Playwright for TUIs" project, this is a significant gap, especially given the concurrency-sensitive async changes in the codebase.

Scope

Test infrastructure

• Add pytest and pytest-asyncio to dev dependencies
• Create tests/ directory structure
• Add conftest.py with shared fixtures (e.g., session launch/teardown)

Unit tests for ScreenSession

• _update_buffer() drain loop behavior
• get_highlighted_regions() edge cases (empty screen, single color, all highlighted)
• get_buffer_display() rendering
• get_char_at() bounds checking
• get_buffer_line() bounds checking

Tool-level integration tests

• launch_tui → send_keys → capture_screen → assert_contains → close_session round trip using example_tui_app.py
• Buffer mode: launch_tui(mode="buffer") → wait_for_text → find_highlighted → assert_at_position
• expect_text finding/timing out
• wait_for_text finding/timing out
• send_ctrl (Ctrl+C)

Async behavior tests

• Verify expect_text doesn't block the event loop
• Verify wait_for_text times out correctly
• Verify send_keys delay arithmetic

Error path tests

• Invalid session ID
• Closed/missing sessions
• Stream-mode-only tools called on buffer sessions and vice versa

CI workflow

• Add .github/workflows/test.yml running pytest on Python 3.10, 3.11, 3.12+

Acceptance criteria

• pytest runs green locally
• CI workflow runs on push and PR
• Coverage for all MCP tools (at least happy path + one error path each)