fix: Replace build-time OpenAPI generated models

[jhateley-godaddy]

Related issue

Fixes #99

Summary

Replace build-time OpenAPI model generation with committed model classes.

The API model classes move from generated code to committed source. The old code came from package com.godaddy.ans.sdk.model.generated, which the OpenAPI generator produced at build time. The new code lives in package com.godaddy.ans.sdk.model. The build no longer generates code. This change removes the org.openapi.generator plugin and its wiring. The api-spec.yaml file stays in the repository as a reference. It is no longer a generator input.

• ans-sdk-api/build.gradle.kts: removed the OpenAPI generator plugin, the openApiGenerate config, the generated source set, and the checkstyle-disable block. Added JUnit and AssertJ test dependencies. - Added the model classes under com.godaddy.ans.sdk.model (agent, catalog, identity, CSR, renewal, and event models).
• Updated the imports in agent-client, discovery, registration, and the Spring Boot example to the new package.

Testing

• Added a unit test for each model class. E2E tests ran against working implementation.

AI assistance

Assisted-by: Claude Code (claude-fable-5)

Checklist

• The PR title follows Conventional Commits — release notes are generated from it
• Tests cover the change
• The linked issue above uses a closing keyword
• Every commit is signed off (git commit -s) certifying the DCO

[bchen-godaddy]

The new model classes are move from the com.godaddy.ans.sdk.model.generated package (produced by org.openapi.generator at compile time) to com.godaddy.ans.sdk.model (committed source).

• They are faithful ports of their previously generated version. Only one functional delta — the removal of toUrlQueryString() from all 55 files is intentional and correct (the method depended on ApiClient, which is now out of scope).

• The generator plugin and all its Gradle wiring are removed. Coverage is thorough for model POJOs.

• All four consuming modules (agent-client, discovery, registration, spring-boot example) are updated to use the new package. Unit tests are added for every model class.

Only a minor finding below.