fix(tui): prevent usize to u16 overflow in interactive renderer

[echobt]

Summary

Fixes #5296 - InteractiveRenderer required_height usize to u16 cast overflow.

Problem

Unchecked cast from usize to u16 can silently truncate values exceeding 65535.

Solution

Used saturating conversion with u16::MAX cap to prevent overflow.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes two critical safety issues: a potential usize to u16 overflow in the TUI renderer and UTF-8 boundary violations in agent mention parsing.

Key Changes:

• renderer.rs: Replaced unsafe as u16 cast with u16::try_from().unwrap_or(u16::MAX) and chained saturating additions to prevent overflow when calculating widget height with large item counts
• mention.rs: Added safe_slice_up_to() and safe_slice_from() helper functions that validate UTF-8 character boundaries before slicing, preventing panics when parsing mentions with multibyte characters (CJK, emoji, etc.)
• Comprehensive test coverage added for UTF-8 edge cases including Japanese text, emojis, and mixed-encoding scenarios

Both fixes are defensive, well-implemented, and include proper test coverage. The changes prevent potential runtime panics without altering the intended behavior.

Confidence Score: 5/5

• This PR is safe to merge with no risks identified
• Both fixes address real safety issues with correct implementations. The overflow protection uses proper Rust patterns (try_from + saturating operations), and the UTF-8 boundary validation follows best practices. Comprehensive test coverage validates all edge cases.
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-tui/src/interactive/renderer.rs	Added saturating conversions to prevent usize to u16 overflow in required_height() calculation
src/cortex-agents/src/mention.rs	Added UTF-8 boundary validation with safe_slice_up_to and safe_slice_from helper functions, comprehensive test coverage included

Sequence Diagram

sequenceDiagram
    participant User
    participant InteractiveWidget
    participant Renderer
    participant MentionParser
    
    Note over User,MentionParser: Overflow Prevention Flow
    
    User->>InteractiveWidget: Request widget with many items
    InteractiveWidget->>InteractiveWidget: Calculate required_height()
    Note over InteractiveWidget: items_count (usize) could exceed u16::MAX
    InteractiveWidget->>InteractiveWidget: u16::try_from(items_count)
    alt items_count > 65535
        InteractiveWidget->>InteractiveWidget: Return u16::MAX
    else items_count <= 65535
        InteractiveWidget->>InteractiveWidget: Return items_count as u16
    end
    InteractiveWidget->>InteractiveWidget: saturating_add(header_height)
    InteractiveWidget->>InteractiveWidget: saturating_add(search_height)
    InteractiveWidget->>InteractiveWidget: saturating_add(hints_height)
    InteractiveWidget->>InteractiveWidget: saturating_add(border_height)
    InteractiveWidget->>Renderer: Return safe height (no overflow)
    
    Note over User,MentionParser: UTF-8 Boundary Safety Flow
    
    User->>MentionParser: Parse text with multibyte chars: "日本語 @general task"
    MentionParser->>MentionParser: find_first_valid_mention()
    MentionParser->>MentionParser: Get mention.start and mention.end
    Note over MentionParser: Byte positions may not align with char boundaries
    MentionParser->>MentionParser: safe_slice_up_to(text, mention.start)
    alt mention.start is char boundary
        MentionParser->>MentionParser: Return &text[..mention.start]
    else mention.start inside multibyte char
        MentionParser->>MentionParser: Search backwards for valid boundary
        MentionParser->>MentionParser: Return &text[..valid_pos]
    end
    MentionParser->>MentionParser: safe_slice_from(text, mention.end)
    alt mention.end is char boundary
        MentionParser->>MentionParser: Return &text[mention.end..]
    else mention.end inside multibyte char
        MentionParser->>MentionParser: Search forwards for valid boundary
        MentionParser->>MentionParser: Return &text[valid_pos..]
    end
    MentionParser->>User: Return parsed result (no panic)

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[greptile-apps]

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[echobt]

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