resolve #810

Summary

• openapi-generator 7.21.0 changed the PHP api.mustache template to use {{paramName}} (camelCase) as form data keys in FormDataProcessor::prepare(), instead of {{baseName}} (snake_case)
• This causes incorrect request bodies (e.g. grantType=client_credentials instead of grant_type=client_credentials), breaking all form-based endpoints
• Add a custom api.mustache override with a single-line fix: '{{baseName}}' instead of '{{paramName}}'

Test plan

• Ran python generate-code.py with 7.21.0 locally and check diff of auto generated codes
• All 23 tests pass (674 assertions), including the previously failing issueStatelessChannelToken tests