fix(tui): prevent usize to u16 overflow in card count displays

[echobt]

Summary

Fixes #5166, #5159, #5141 - Card count displays overflow on large values.

Problem

Direct usize to u16 casts truncate when counts exceed 65535.

Solution

Used saturating conversion to cap at u16::MAX.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes overflow issues in card display height calculations and UTF-8 string slicing panics.

TUI Card Changes (3 files)

• Replaced direct usize as u16 casts with safe u16::try_from().unwrap_or(u16::MAX) conversion
• Added saturating_add() when adding padding to prevent arithmetic overflow
• Affects desired_height() calculations in commands, models, and sessions cards

Mention Parsing Changes

• Added safe_slice_up_to() and safe_slice_from() helper functions to handle UTF-8 character boundaries
• Prevents panics when slicing strings at invalid byte positions (inside multibyte characters)
• Includes comprehensive test coverage for ASCII, CJK characters, and emojis

Confidence Score: 5/5

• Safe to merge - defensive programming fixes with comprehensive tests
• All changes use proper Rust safety patterns (try_from, saturating operations, UTF-8 boundary checks) and include extensive test coverage for edge cases
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-tui/src/cards/commands.rs	replaced unsafe as u16 cast with safe try_from().unwrap_or(u16::MAX) and added saturating_add for overflow protection
src/cortex-tui/src/cards/models.rs	replaced unsafe as u16 cast with safe try_from().unwrap_or(u16::MAX) and added saturating_add for overflow protection
src/cortex-tui/src/cards/sessions.rs	replaced unsafe as u16 cast with safe try_from().unwrap_or(u16::MAX) and added saturating_add for overflow protection
src/cortex-agents/src/mention.rs	added UTF-8 boundary validation helpers (safe_slice_up_to, safe_slice_from) to prevent panics when slicing multibyte characters, with comprehensive tests

Sequence Diagram

sequenceDiagram
    participant User
    participant TUI as TUI Cards
    participant Conversion as Type Conversion
    participant Display as Display Rendering
    
    User->>TUI: View card (commands/models/sessions)
    TUI->>TUI: Get collection length (usize)
    TUI->>Conversion: u16::try_from(len)
    alt len <= u16::MAX
        Conversion-->>TUI: Ok(len as u16)
    else len > u16::MAX
        Conversion-->>TUI: Err(_)
        TUI->>TUI: Use u16::MAX instead
    end
    TUI->>TUI: saturating_add(count, padding)
    TUI->>Display: Calculate desired_height
    Display-->>User: Render card with safe height

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[echobt]

Consolidated into #69 - fix(tui): consolidated TUI fixes for overflow, positioning, and panic prevention