fix(engine): use saturating casts in git_info to prevent overflow

[echobt]

Summary

Fixes #5185 - git_info.rs uses unchecked narrowing casts.

Problem

Several type conversions can silently truncate large values.

Solution

Applied saturating conversions to prevent silent truncation.

[greptile-apps]

Greptile Overview

Greptile Summary

Applied saturating type conversions across three modules to prevent overflow and panic scenarios when converting between usize, u32, and u16 types, and when slicing UTF-8 strings.

Key Changes:

• git_info.rs: Replaced direct as u32 casts with .min(u32::MAX as usize) as u32 for counting git changes, untracked files, and stash indices
• renderer.rs: Used u16::try_from().unwrap_or(u16::MAX) and saturating_add for UI height calculations to prevent overflow in terminal rendering
• mention.rs: Added safe_slice_up_to and safe_slice_from helper functions to validate UTF-8 boundaries before slicing, preventing panics with multi-byte characters (emojis, CJK text, etc.)

The changes follow Rust safety best practices by ensuring operations clamp to maximum values rather than silently truncating or panicking. The mention.rs changes include comprehensive test coverage for UTF-8 edge cases.

Confidence Score: 5/5

• This PR is safe to merge with no concerns
• All changes correctly implement saturating conversions to prevent overflow. The code follows Rust best practices, includes comprehensive test coverage for UTF-8 edge cases, and only affects type safety without changing business logic
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-engine/src/git_info.rs	Applied saturating casts to prevent overflow when counting git changes and stash indices
src/cortex-tui/src/interactive/renderer.rs	Used try_from with saturating addition to safely handle height calculations and prevent u16 overflow
src/cortex-agents/src/mention.rs	Added UTF-8 boundary validation helpers and comprehensive tests to prevent panics when slicing strings with multi-byte characters

Sequence Diagram

sequenceDiagram
    participant User
    participant GitInfo
    participant TUI
    participant MentionParser
    
    Note over GitInfo,MentionParser: Overflow Prevention Flow
    
    User->>GitInfo: Get repository status
    GitInfo->>GitInfo: Count changed files (usize)
    GitInfo->>GitInfo: Apply .min(u32::MAX as usize) as u32
    GitInfo-->>User: Return safe u32 count
    
    User->>GitInfo: List stashes
    GitInfo->>GitInfo: Enumerate stashes (usize)
    GitInfo->>GitInfo: Apply i.min(u32::MAX as usize) as u32
    GitInfo-->>User: Return safe u32 index
    
    User->>TUI: Calculate widget height
    TUI->>TUI: Get items_count (usize)
    TUI->>TUI: Apply u16::try_from().unwrap_or(u16::MAX)
    TUI->>TUI: Use saturating_add for height components
    TUI-->>User: Return safe u16 height
    
    User->>MentionParser: Parse mention with UTF-8 text
    MentionParser->>MentionParser: Check is_char_boundary(pos)
    alt Invalid boundary
        MentionParser->>MentionParser: Find nearest valid boundary
    end
    MentionParser-->>User: Return valid string slice

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[greptile-apps]

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[echobt]

Consolidated into #71 - fix: consolidated numeric overflow/underflow prevention with saturating operations