fix(cli): use safe UTF-8 slicing for session IDs in lock command

[echobt]

Summary

Fixes #5276 - Lock command session ID slicing panics on multi-byte UTF-8.

Problem

Session ID handling uses byte-based string slicing at 4 locations.

Solution

Replaced with safe slicing using .get() or char-aware methods.

Added a helper function safe_char_prefix that extracts string prefixes by character count rather than byte count, preventing panics on multi-byte UTF-8 characters.

Changes

• Added safe_char_prefix helper function for UTF-8 safe slicing
• Fixed 4 locations in is_session_locked, run_list, and run_check functions
• Added comprehensive unit tests for the helper function

[greptile-apps]

Greptile Overview

Greptile Summary

Replaced unsafe byte-based string slicing with character-aware slicing to prevent panics on multi-byte UTF-8 characters in session IDs.

• Added safe_char_prefix() helper function that uses char_indices() to slice strings by character count rather than byte count
• Updated 4 locations where [..8.min(len)] slicing was used: is_session_locked() (line 168), run_list() (line 320), and run_check() (lines 344 and 354)
• Added comprehensive unit tests covering ASCII, multi-byte UTF-8 (emojis, Chinese characters), and boundary cases

Confidence Score: 5/5

• This PR is safe to merge with no risk
• The fix correctly addresses a real UTF-8 panic bug with a proper solution using character-aware slicing, includes comprehensive test coverage for edge cases, and makes no behavioral changes to the lock logic
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-cli/src/lock_cmd.rs	Replaced unsafe byte-based slicing with character-aware helper function, added comprehensive UTF-8 tests

Sequence Diagram

sequenceDiagram
    participant User
    participant LockCmd as Lock Command
    participant SafePrefix as safe_char_prefix()
    participant LockFile as Lock File Storage

    Note over User,LockFile: Session ID Display/Matching Flow

    User->>LockCmd: list command
    LockCmd->>LockFile: load_lock_file()
    LockFile-->>LockCmd: locked_sessions[]
    
    loop For each locked session
        LockCmd->>SafePrefix: safe_char_prefix(session_id, 8)
        SafePrefix->>SafePrefix: char_indices().nth(8)
        SafePrefix->>SafePrefix: slice at byte boundary
        SafePrefix-->>LockCmd: safe 8-char prefix
        LockCmd->>User: display short_id
    end

    User->>LockCmd: check/is_locked(session_id)
    LockCmd->>LockFile: load_lock_file()
    LockFile-->>LockCmd: locked_sessions[]
    
    loop For each entry
        LockCmd->>SafePrefix: safe_char_prefix(entry.session_id, 8)
        SafePrefix-->>LockCmd: safe 8-char prefix
        LockCmd->>LockCmd: starts_with() check
    end
    
    LockCmd-->>User: lock status result

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[greptile-apps]

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[echobt]

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