Fix 4260 windows slash input

[LIHUA919]

Problem

Windows users cannot input the slash character "/" in Codex CLI, preventing use of slash commands.

Root Cause

The textarea input handler only accepts characters with KeyModifiers::NONE | KeyModifiers::SHIFT, but Windows keyboard layouts may send slash with other modifiers.

Solution

Add special handling for slash character to accept any modifier combination:

KeyEvent {
    code: KeyCode::Char('/'),
    modifiers: _, // Accept any modifiers for slash
    ..
} => self.insert_str("/"),

Testing

• ✅ Compilation: cargo check --package codex-tui
• ✅ Unit tests: 22/22 textarea tests passed
• ✅ No impact on other character input

Impact

• Fixes Windows slash command input issue
• Minimal change (8 lines added)
• Backward compatible

Fixes #4260

[github-actions]

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[esdrasrenan]

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