fix stale IGM cache in node pool

[c2thorn]

Fixes hashicorp/terraform-provider-google#27966
Splits #18397

This PR fixes a bug where stale Instance Group Manager target sizes were served from the client-side provider cache after GKE node count updates or when ignore_node_count_changes was active.

To solve stale IGM target sizes, this PR:

1. Adds an invalidateItem(igmUrl) method to instanceGroupManagerCache to allow removing specific IGM entries.
2. Calls the invalidation inside flattenNodePool when ignore_node_count_changes is active, as the cache isn't needed if we are ignoring the count.
3. Calls invalidation in nodePoolUpdate after a size update, ensuring subsequent reads fetch fresh GCE API data.

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

container: fixed a bug where `google_container_node_pool` and `google_container_cluster` failed to invalidate their Instance Group Manager cache when a resize occurred or when `ignore_node_count_changes` was active

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit f16989d:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	1 file changed, 19 insertions(+)
google-beta provider	View Diff	1 file changed, 19 insertions(+)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
320	304	15	1

Affected Service Packages

• container

Learn how VCR tests work

---

Step 1: Replaying Mode

Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit.

Click here to see the affected tests

• TestAccContainerCluster_regionalWithNodePool

View the replaying VCR build log

---

Step 2: Recording Mode

Recording Mode	Replaying Rerun	Test Name
❌ Error · Log	-	TestAccContainerCluster_regionalWithNodePool

Caution

Issues requiring attention before PR completion

🔴 Initial Recording Failed: Some tests failed during the recording step. See the table above for details.

Please address these issues to complete your PR. If you believe these detections are incorrect or unrelated to your change, please raise the concern with your reviewer.

View the recording VCR build log or the debug logs folder for detailed results.

@c2thorn VCR tests complete for f16989d!

[c2thorn]

TestAccContainerCluster_regionalWithNodePool will pass after #18453 is merged