Fix memory leaks in validate_function.c

[matapatos]

Fix memory leaks in validate_function.c

Changes

• Track field_name ownership with field_name_owned flag to prevent double-free and memory leaks
• Release field_name consistently in all error paths (exception handling, stop_at_first_error, missing fields)
• Fix add_field_error to use ZVAL_STRINGL instead of ZVAL_STRING + zval_ptr_dtor (redundant since hash add does its own refcounting)
• Ensure errors array is always cleaned up on all exception paths
• Add continue statement after handling missing field_value to prevent fallthrough

Impact

• Prevents memory leaks when validation fails
• Improves code consistency and maintainability
• No functional changes to validation logic

Closes #1