Fix: prevent crash on legacy X10 mouse reports after terminal rebuild

[Jyotish08]

closes #6668 ### Problem
When a terminal emulator is torn down and rebuilt underneath a running Textual application (e.g. VS Code terminal reload, tmux detach/reattach, SSH reconnect):

1. The rebuilt terminal resets its negotiation and falls back to legacy X10 mouse reporting (ESC [ M Cb Cx Cy).
2. When the pointer moves past column ~95 (Cx > 127), the raw coordinate bytes exceed 0x7F (non-ASCII).
3. The input thread's strict UTF-8 incremental decoder raises UnicodeDecodeError, causing the input loop and application to crash.
4. If the crash is avoided, mouse tracking remains broken because the terminal stays in legacy X10 encoding instead of SGR mode (?1006).

Solution

1. Fault-Tolerant Input Decoding: Updated incremental UTF-8 decoders in linux_driver.py, linux_inline_driver.py, and web_driver.py to use errors="replace". This prevents UnicodeDecodeError when raw non-ASCII X10 mouse bytes (> 0x7F) are read from stdin.
2. Automatic SGR Mouse Mode Re-assertion:
   • Added LegacyMouseReport message class in messages.py.
   • Updated XTermParser in _xterm_parser.py to emit LegacyMouseReport upon encountering an X10 mouse sequence (ESC [ M ...).
   • Updated Driver.process_message to handle LegacyMouseReport by invoking self._enable_mouse_support(), re-asserting SGR mouse mode (\x1b[?1006h) to restore full mouse tracking.
3. Tests:
   • Added test_legacy_x10_mouse_report_emits_legacy_mouse_report in tests/test_xterm_parser.py.
   • Added test_driver_reasserts_mouse_on_legacy_report in tests/test_driver.py.

[Copilot]

Pull request overview

This PR hardens Textual’s input pipeline against legacy X10 mouse reporting that can appear after a terminal teardown/rebuild (e.g., VS Code terminal reload), preventing UTF-8 decode crashes and restoring full mouse tracking by re-enabling SGR mouse mode when X10 sequences are detected.

Changes:

• Switch stdin byte-stream decoding in terminal/web drivers to a fault-tolerant UTF-8 incremental decoder (errors="replace") to avoid UnicodeDecodeError on non-UTF-8 X10 coordinate bytes.
• Introduce a LegacyMouseReport message and teach XTermParser to emit it when ESC [ M ... (X10) is seen, prompting the driver to re-assert mouse mode via _enable_mouse_support().
• Add targeted tests covering parser emission and driver behavior on legacy X10 reports.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_xterm_parser.py	Adds regression test ensuring X10 sequences emit LegacyMouseReport.
tests/test_driver.py	Adds regression test ensuring the driver re-enables mouse support on LegacyMouseReport.
src/textual/messages.py	Introduces the LegacyMouseReport message type.
src/textual/drivers/web_driver.py	Makes UTF-8 decoding tolerant (errors="replace") in the web input loop.
src/textual/drivers/linux_inline_driver.py	Makes UTF-8 decoding tolerant (errors="replace") in the inline terminal input loop.
src/textual/drivers/linux_driver.py	Makes UTF-8 decoding tolerant (errors="replace") in the terminal input loop.
src/textual/driver.py	Handles LegacyMouseReport by re-enabling mouse support from the input thread.
src/textual/_xterm_parser.py	Detects X10 mouse sequences and emits LegacyMouseReport prior to mouse parsing.

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