Strip ANSI and OSC escape sequences from untrusted terminal output - #5928

Untrusted terminal output reaches the terminal unfiltered: command output is printed with plain print() in run_cmd_subprocess(), and tool/assistant messages go to Rich as Text, which passes CSI and OSC bytes through. A file from an untrusted checkout, shown via a command or quoted in model output, can clear the screen or print a fake confirmation prompt.

• aider/utils.py: strip_ansi helper covering CSI (ESC[...letter) and OSC (ESC]...BEL / ESC]...ESC), plus AnsiStripper for sequences split across reads.
• aider/run_cmd.py: run_cmd_subprocess() prints and returns stripped output.
• aider/io.py: InputOutput._tool_message() and assistant_output() strip message text before console.print. Rich style arguments are unchanged, so aider's own colors are unaffected. Code blocks quoting escape bytes lose those bytes in display.
• tests/basic/test_utils.py, tests/basic/test_run_cmd.py: helper cases, split-sequence cases, and stripped run_cmd output.

Tests:

• pytest tests/basic/test_utils.py tests/basic/test_run_cmd.py: 7 passed
• pytest tests/basic/test_io.py: 23 passed

Notes:

• Public hardening, not a validated vulnerability.
• Aider Individual CLA signature is still owed before merge; filing is not blocked by it, merge is.
• The pexpect interactive path is out of scope.
• No live spoofing demo against aider itself was run.