DEV-1874: Fix: remove single-value enum from error code fields (PHP SDK generation)

[ShreyaThapa]

This PR replaces single-value enum constraints on a few error code fields with plain string + example. Speakeasy was generating PHP string-backed enums for these codes, which then triggered PHPStan failures due to (int) $code casts in generated toException() methods.

Why

• Single-value enums for error codes don’t add meaningful validation value, but they change SDK typing (enum vs string).
• In the generated PHP SDK, this resulted in PHPStan errors like “Cannot cast … to int” when building exception objects.

Impact / Compatibility

• No API behavior change (response payloads unchanged).
• Improves SDK generation compatibility, especially for PHP; reduces likelihood of downstream type/cast issues.
  Other SDKs should be unaffected or improved (strings are broadly compatible).