fix(engine): handle semaphore and init failures gracefully in async_utils

[echobt]

Summary

Fixes #5205 and #5201 - Potential panics in async utilities.

Problem

Semaphore acquire and AsyncOnce init use unwrap() that can panic:

• ConcurrencyLimiter::execute in async_utils.rs panics if semaphore is closed
• AsyncOnce::get_or_init panics if value is missing after init flag is set
• concurrent() function panics if semaphore is closed
• ConcurrencyLimiter::acquire in ratelimit.rs panics if semaphore is closed

Solution

Added proper error handling with Result return types:

• ConcurrencyLimiter::execute now returns Result<T> instead of T
• AsyncOnce::get_or_init now returns Result<T> with descriptive error messages
• concurrent() function now returns Result<Vec<T>>
• ConcurrencyLimiter::acquire in ratelimit.rs now returns Result<ConcurrencyPermit>

All semaphore acquire calls now use map_err instead of unwrap() to provide descriptive error messages when semaphores are closed unexpectedly.

Testing

• All existing tests updated to handle new Result types
• All tests pass: cargo test -p cortex-engine async_utils and cargo test -p cortex-engine ratelimit

[greptile-apps]

Greptile Overview

Greptile Summary

This PR eliminates potential panic points in async utilities by replacing unwrap() calls with proper error handling.

Key Changes:

• async_utils.rs: Converted ConcurrencyLimiter::execute(), AsyncOnce::get_or_init(), and concurrent() to return Result<T> instead of panicking on semaphore closure or inconsistent state
• ratelimit.rs: Updated ConcurrencyLimiter::acquire() to return Result<ConcurrencyPermit> with proper error handling
• mention.rs: Added UTF-8 boundary validation helpers (safe_slice_up_to, safe_slice_from) to prevent panics when slicing strings at invalid byte positions
• TUI files: Fixed potential overflow issues in desired_height() calculations by using try_from() and saturating_add() instead of unsafe as casts

All error cases now return descriptive CortexError::Internal variants instead of panicking. Tests have been updated to handle the new Result types appropriately.

Confidence Score: 5/5

• This PR is safe to merge - it improves robustness without breaking existing functionality
• All changes replace panic-inducing code with proper error handling, tests are updated correctly, and the changes are well-documented with clear error messages
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-engine/src/async_utils.rs	Replaced unwrap() calls with proper error handling using Result types for semaphore operations
src/cortex-engine/src/ratelimit.rs	Added error handling for semaphore acquire operation in ConcurrencyLimiter
src/cortex-agents/src/mention.rs	Added UTF-8 boundary validation for string slicing operations to prevent panics

Sequence Diagram

sequenceDiagram
    participant Caller
    participant ConcurrencyLimiter
    participant Semaphore
    participant ErrorHandler

    Note over Caller,ErrorHandler: Before: Panics on semaphore closure
    Caller->>ConcurrencyLimiter: execute(fn)
    ConcurrencyLimiter->>Semaphore: acquire().await
    alt Semaphore closed (old behavior)
        Semaphore-->>ConcurrencyLimiter: Err
        ConcurrencyLimiter->>ConcurrencyLimiter: unwrap() → PANIC!
    else Semaphore open (old behavior)
        Semaphore-->>ConcurrencyLimiter: Ok(permit)
        ConcurrencyLimiter->>Caller: fn().await result
    end

    Note over Caller,ErrorHandler: After: Graceful error handling
    Caller->>ConcurrencyLimiter: execute(fn)
    ConcurrencyLimiter->>Semaphore: acquire().await
    alt Semaphore closed (new behavior)
        Semaphore-->>ConcurrencyLimiter: Err
        ConcurrencyLimiter->>ErrorHandler: map_err → CortexError::Internal
        ErrorHandler-->>Caller: Result::Err
    else Semaphore open (new behavior)
        Semaphore-->>ConcurrencyLimiter: Ok(permit)
        ConcurrencyLimiter->>Caller: Result::Ok(fn().await)
    end

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[greptile-apps]

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[echobt]

Consolidated into #73 - fix: consolidated error handling improvements