Add TaskAction validation before finalizer registration

[AdilFayyaz]

Tracking issue

Closes #6934

Why are the changes needed?

In PR #6903, it was noted that plugin resolution (the check for whether a registered plugin exists for a given taskType) happened after the finalizer was added to the resource. This meant that if a TaskAction had an unrecognized taskType, the controller would:

1. Add a finalizer on the first reconcile
2. Discover the plugin is missing on the second reconcile
3. Mark the resource terminal

At that point, deleting the resource requires the controller to run handleAbortAndFinalize to remove the finalizer even though no plugin ever ran and there is nothing to clean up. Additionally, validateSpec only checked 4 of the 9 required spec fields, leaving runName, org, project, domain, and actionName unvalidated at the controller level.

What changes were proposed in this pull request?

• Replaced the separate validateSpec function and inline ResolvePlugin block with a unified validateTaskAction function that validates all required spec fields and resolves the plugin in a single pass
• This validation now runs before the finalizer is added if validation fails, the resource is marked terminal (Failed condition), not requeued, and no finalizer is registered, making deletion trivial
• Introduced a pluginResolver interface so validateTaskAction is testable without a real plugin registry
• Added taskaction_validation_test.go with standard Go unit tests covering all 9 required field checks and the plugin-not-found case
• Fixed the existing integration test fixture which was missing required fields (taskType, taskTemplate)

How was this patch tested?

• Unit tests: go test ./pkg/controller/... -run TestValidateTaskAction -v covers all 9 missing-field cases and plugin-not-found
• Integration test: go test ./pkg/controller/... -v confirms reconcile completes without error and sets conditions

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

• main
  • Flyte 2 WIP #6583
    • Add TaskAction validation before finalizer registration #7006 👈