[E2E SDK release] ensure the spec PR is the one that triggers SDK generation.

Scenario:
- [Release plan 36142](https://azsdk-releaseplan-dashboard-hveph5aqhhcfhtgu.westus-01.azurewebsites.net/?filter=36142) was triggered based on [the HCP onboarding PR targeting 2026-06-01-preview (PR 45197)](https://github.com/Azure/azure-rest-api-specs/pull/45197).
- Before SDK release completed, a subsequent [PR introducing 2026-09-01-preview (PR 45624)](https://github.com/Azure/azure-rest-api-specs/pull/45624) was merged and updated the service's default API version.
- The release plan retriggered SDK generation without explicitly pinning the originally intended API version when the plan is created. As a result, generation picked up the new default version (2026-09-01-preview) rather than the originally planned version (2026-06-01-preview).

Suspected gap:
Release plans do not explicitly persist and enforce the API version selected when the plan is created, making them susceptible to changes in the service's default API version before generation occurs.

Proposed fix:
- Either release plan explicitly persist and enforce the API version selected when the plan is created. Do not re-triggerring SDK generation when changes in the service's default API version is merged.
- Or before the release pipeline is triggered, the release plan is susceptible to changes in the service's default API version. And it should call out clearly which API version the SDK is generating upon.