Add completion thread control and script argument passing to runScriptFileAsync

[Copilot]

Extends runScriptFileAsync API with two capabilities: completion handler thread control and runtime script parameterization via string arguments.

Changes

Thread Control

• Added runOnMainThread parameter (defaults to YES for backward compatibility)
• Eliminates thread context switch overhead when UI updates aren't needed
• All error paths respect the thread preference

Script Arguments

• Added argument parameter to pass strings to scripts
• Argument accessible in JavaScript as __scriptArgument global variable
• Set in V8 context before script execution

Backward Compatibility

• Existing method signatures delegate to extended version with defaults
• All existing code continues to work unchanged

API

// Full signature
- (void)runScriptFileAsync:(NSString*)filePath
                  argument:(NSString*)argument
          runOnMainThread:(BOOL)runOnMainThread
                completion:(void(^)(id result, NSError* error))completion;

// Convenience wrappers (maintain existing behavior)
- (void)runScriptFileAsync:(NSString*)filePath completion:(void(^)(id, NSError*))completion;
- (void)runScriptFileAsync:(NSString*)filePath runOnMainThread:(BOOL)runOnMainThread completion:(void(^)(id, NSError*))completion;

Usage

// Background completion for data processing
[runtime runScriptFileAsync:@"process.js"
                    argument:@"{\"userId\":123}"
            runOnMainThread:NO
                  completion:^(id result, NSError* error) {
    // Runs on background thread
}];

// process.js
const data = JSON.parse(__scriptArgument);
return { userId: data.userId, status: 'completed' };

Tests

Added 11 test cases covering:

• Background thread completion with thread verification via NSThread.isMainThread
• Simple string arguments, JSON parsing, nil handling
• Mixed threading scenarios

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