Add tests for scrollback buffer retrieval and viewing

[andrewgazelka]

Summary

Need comprehensive tests for the scrollback buffer functionality to ensure reliable buffer retrieval and viewing.

Tests Needed

• Test buffer captures output correctly
• Test buffer retrieval via socket API (GetScrollback request)
• Test buffer content after various terminal operations
• Test ANSI escape code stripping
• Test buffer size limits and truncation
• Integration test: tap client fetching buffer from running session

[andrewgazelka]

Already tested in scrollback.rs:

• Test buffer captures output correctly (test_push_simple_text, test_push_with_newlines)
• Test ANSI escape code stripping (test_strips_ansi_escapes)
• Test cursor position tracking (test_cursor_position)
• Test getting last N lines (test_get_last_n_lines)

Still needs:

• Test buffer retrieval via socket API (integration test)
• Test buffer size limits and truncation
• Full integration test with tap client