Enforce id validation for WorkloadIdentityPool Managed Identity

[stevenyang72]

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iambeta: enforced `workload_identity_pool_managed_identity_id` field validation per the documented specifications

Add back ID validation on WorkloadIdentityPool Managed Identity. Context: #14110 (comment)

[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 2 insertions(+), 2 deletions(-))
google-beta provider: Diff ( 5 files changed, 63 insertions(+), 10 deletions(-))

[modular-magician]

Tests analytics

Total tests: 37
Passed tests: 31
Skipped tests: 2
Affected tests: 4

Click here to see the affected service packages

• iambeta

#### Action taken

Found 4 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

• TestAccIAMBetaWorkloadIdentityPoolNamespace_full
• TestAccIAMBetaWorkloadIdentityPoolNamespace_iamWorkloadIdentityPoolNamespaceBasicExample
• TestAccIAMBetaWorkloadIdentityPoolNamespace_iamWorkloadIdentityPoolNamespaceFullExample
• TestAccIAMBetaWorkloadIdentityPoolNamespace_minimal

Get to know how VCR tests work

[modular-magician]

🟢 Tests passed during RECORDING mode:
TestAccIAMBetaWorkloadIdentityPoolNamespace_full [Debug log]
TestAccIAMBetaWorkloadIdentityPoolNamespace_iamWorkloadIdentityPoolNamespaceBasicExample [Debug log]
TestAccIAMBetaWorkloadIdentityPoolNamespace_iamWorkloadIdentityPoolNamespaceFullExample [Debug log]
TestAccIAMBetaWorkloadIdentityPoolNamespace_minimal [Debug log]

🟢 No issues found for passed tests after REPLAYING rerun.

---

🟢 All tests passed!

View the build log or the debug log for each test

[github-actions]

@roaks3 This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[roaks3]

Seems reasonable consider the context, noting that new validation could technically impact some existing configs, but it seems unlikely that there would be values failing this validation while succeeding previously