fix(tui): warn on unsupported iTerm2 pet versions

[fcoury-oai]

Why

Older iTerm2 builds can be detected as supporting the image transport that terminal pets use, but in practice they fail to render the pet flow correctly. Instead of silently attempting image rendering, Codex should tell the user that their iTerm2 version is too old and that upgrading is the fix.

What Changed

• gate iTerm2 pet auto-detection on version 3.6.0 or newer
• show a dedicated upgrade message for older or unknown iTerm2 versions instead of the generic unsupported-terminal warning
• keep the existing generic unsupported-terminal path for non-iTerm terminals
• add regression coverage for iTerm2 version parsing and the old-iTerm warning path

How to Test

1. Start Codex in iTerm2 3.6 or newer.
2. Run /pets.
3. Confirm the pets picker opens instead of showing a warning.
4. Start Codex in an older iTerm2 build, or exercise the equivalent test path.
5. Run /pets.
6. Confirm Codex warns that pets require iTerm2 3.6 or newer and tells the user to upgrade.
7. Also verify that a non-iTerm unsupported terminal still shows the generic unsupported-terminal message.

Targeted tests:

• cargo test -p codex-terminal-detection
• cargo test -p codex-tui pets::
• cargo test -p codex-tui slash_pets_on_unsupported_terminal
• cargo test -p codex-tui slash_pets_on_old_iterm2

[canvrno-oai]

Tested and reviewed. When running the /pets command on an older version of iTerm, the expected message was displayed:
⚠ Pets require iTerm2 3.6 or newer. Upgrade iTerm2 to use terminal pets.

When running in a newer iTerm version, /pets worked as expected.