chore: use PHPStan error identifier instead of regex pattern (#389)

## Summary

This PR updates the PHPStan configuration to use an error identifier
instead of a regex pattern for ignoring array type errors in tests.

## Changes

- Replace regex pattern  with identifier 

## Benefits

Using error identifiers is the recommended approach because:
- **More stable**: Identifiers don't change across PHPStan versions,
unlike error message text
- **Cleaner**: Easier to read and understand the intent
- **Maintainable**: Less prone to breaking if error message wording
changes

## Testing

PHPStan analysis passes with the new configuration.

Author: OskarStark

src/agent/phpstan.dist.neon

@@ -5,6 +5,6 @@ parameters:
         - tests/
     ignoreErrors:
         -
-            message: '#no value type specified in iterable type array#'
+            identifier: missingType.iterableValue
             path: tests/*