Add timeout support to ReadSignal.until method

[Copilot]

This PR adds an optional timeout parameter to the ReadSignal.until method to prevent memory leaks and provide better control over asynchronous signal waiting.

Background

In issue #124, a user created an extension method that included timeout functionality for ReadSignal, noting that the existing until method was missing timeout support. The library maintainer confirmed this gap and promised to add timeout functionality. The user specifically highlighted that using .timeout() wrapper on the future would result in leaked Effects that never get disposed.

Changes

Core Implementation

Added an optional Duration? timeout parameter to the until method:

FutureOr<T> until(
  bool Function(T value) condition, {
  Duration? timeout,  // New optional parameter
})

The implementation:

• Maintains full backward compatibility - existing code works unchanged
• Uses a Timer to complete with TimeoutException when timeout expires
• Properly disposes both the Effect and Timer to prevent memory leaks
• Returns immediately when the condition is already met (regardless of timeout)

Usage Examples

// Without timeout (existing behavior)
await signal.until((value) => value > 10);

// With timeout (new feature)  
try {
  final result = await signal.until(
    (value) => value > 10,
    timeout: Duration(seconds: 5),
  );
  print('Condition met: $result');
} catch (TimeoutException) {
  print('Timeout after 5 seconds');
}

Testing

Added comprehensive tests covering:

• Condition met before timeout
• Timeout occurs before condition is met
• Condition already met (immediate return)
• Proper cleanup on timeout
• Proper cleanup on success

All 148 tests pass (143 existing + 5 new timeout tests).

Documentation

Updated the API documentation in docs/api-docs/read-signal.mdx to include the new timeout parameter with usage examples.

Benefits

1. Prevents memory leaks - proper cleanup of Effects and Timers
2. Better error handling - clear TimeoutException when timeout occurs
3. Backward compatible - no breaking changes to existing code
4. Integrated solution - no need for external timeout wrappers that leak resources

Fixes #124.

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