fix(resume): use char-aware string truncation for UTF-8 safety

[echobt]

Summary

Fixes #5288 - Path truncation panics on multi-byte UTF-8.

Problem

String truncation used byte-based slicing that panics on multi-byte characters.

Solution

Use char-based iteration for safe truncation.

[greptile-apps]

Greptile Overview

Greptile Summary

Fixed critical UTF-8 safety bugs that caused panics when slicing multi-byte characters. Replaced unsafe byte-based string slicing with char-aware methods across three files:

• resume_picker.rs: Replaced .len() with .chars().count() and byte slicing with .chars().take() in truncate_string() function
• notification.rs: Used char_indices() with .get() for safe session ID truncation at char boundaries
• import_cmd.rs: Wrapped all string slicing operations in .get() to gracefully handle invalid UTF-8 boundaries when extracting base64 data

All changes maintain original functionality while preventing runtime panics on multi-byte UTF-8 input (emoji, CJK characters, etc.). Comprehensive unit tests added for UTF-8 edge cases.

Confidence Score: 5/5

• This PR is safe to merge with no risk - it fixes critical safety bugs without changing behavior
• All changes follow Rust best practices for UTF-8 handling, comprehensive tests verify correctness, and the fixes only improve safety without altering logic
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-resume/src/resume_picker.rs	Fixed UTF-8 panic in truncate_string by using char-based iteration instead of byte slicing, with comprehensive tests
src/cortex-cli/src/utils/notification.rs	Replaced byte-based string slicing with char-aware boundary detection for session ID truncation
src/cortex-cli/src/import_cmd.rs	Added safe slicing with .get() to prevent panics when extracting base64 data from message content

Sequence Diagram

sequenceDiagram
    participant User
    participant ResumePicker as Resume Picker UI
    participant Notification as Notification System
    participant ImportCmd as Import Command
    participant TruncateString as truncate_string()
    participant SafeSlicing as Safe UTF-8 Slicing

    Note over ResumePicker,SafeSlicing: String Truncation Flow (resume_picker.rs)
    User->>ResumePicker: Display session with path/title
    ResumePicker->>TruncateString: truncate_string(text, width)
    TruncateString->>TruncateString: Count chars (not bytes)
    alt char_count <= width
        TruncateString-->>ResumePicker: Return original string
    else width > 3
        TruncateString->>TruncateString: Take (width-3) chars safely
        TruncateString-->>ResumePicker: Return truncated + "..."
    else
        TruncateString->>TruncateString: Take width chars
        TruncateString-->>ResumePicker: Return truncated string
    end
    ResumePicker-->>User: Display safe truncated text

    Note over Notification,SafeSlicing: Session ID Truncation (notification.rs)
    User->>Notification: Task completes (session_id)
    Notification->>SafeSlicing: Truncate session_id to 8 chars
    SafeSlicing->>SafeSlicing: char_indices().take_while(idx < 8)
    SafeSlicing->>SafeSlicing: Find last valid char boundary
    SafeSlicing->>SafeSlicing: Use .get(..end) for safe slice
    SafeSlicing-->>Notification: Return safe truncated ID
    Notification-->>User: Show desktop notification

    Note over ImportCmd,SafeSlicing: Base64 Extraction (import_cmd.rs)
    User->>ImportCmd: Import messages with embedded images
    ImportCmd->>ImportCmd: Find "data:image/" pattern
    ImportCmd->>SafeSlicing: Slice at data_uri_start with .get()
    alt Valid byte boundary
        SafeSlicing-->>ImportCmd: Return Some(slice)
        ImportCmd->>SafeSlicing: Slice at base64_start with .get()
        alt Valid byte boundary
            SafeSlicing-->>ImportCmd: Return Some(base64_data)
            ImportCmd->>ImportCmd: Validate base64 encoding
        else Invalid boundary
            SafeSlicing-->>ImportCmd: Return None
            ImportCmd->>ImportCmd: Skip (continue)
        end
    else Invalid boundary
        SafeSlicing-->>ImportCmd: Return None
        ImportCmd->>ImportCmd: Skip message (continue)
    end
    ImportCmd-->>User: Safe import without panics

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[greptile-apps]

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[echobt]

Consolidated into #70 - fix: consolidated UTF-8 safety improvements for string slicing