fix(php): set XDEBUG_MODE=coverage for PHPUnit 11 compatibility

PHPUnit 11 returns exit code 1 when xdebug coverage mode isn't set,
even if all tests pass. This caused false failures in the QA pipeline.

Setting XDEBUG_MODE=coverage in the test environment resolves the
warning and ensures tests return exit code 0 on success.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/php/testing.go

@@ -87,6 +87,9 @@ func RunTests(ctx context.Context, opts TestOptions) error {
 	cmd.Stderr = opts.Output
 	cmd.Stdin = os.Stdin
 
+	// Set XDEBUG_MODE=coverage to avoid PHPUnit 11 warning
+	cmd.Env = append(os.Environ(), "XDEBUG_MODE=coverage")
+
 	return cmd.Run()
 }