container: fix ignore_node_count_changes flattener permadiff

[c2thorn]

Fixes hashicorp/terraform-provider-google#27946
Splits #18397

The failing test was caused by a real permadiff where the ignore_node_count_changes flag was not persisted in google_container_cluster.node_pool.

Unlike top-level fields in google_container_node_pool which automatically persist their state in ResourceData during Read, unset nested fields in google_container_cluster.node_pool blocks get nulled out during Read.

To resolve the permadiff and fix the test, this PR:

1. Explicitly maps ignore_node_count_changes from ResourceData inside GKE's flattenNodePool using the block prefix.
2. Adds node_pool.0.managed_instance_group_urls to ImportStateVerifyIgnore in TestAccContainerCluster_regionalWithNodePool. Since virtual fields are not loaded during import, ignore_node_count_changes defaults to false and the API populates managed_instance_group_urls, causing an import verify diff.

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container: fixed a permadiff where `ignore_node_count_changes` was not persisted in `google_container_cluster.node_pool`

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit e3a0545:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	3 files changed, 13 insertions(+), 4 deletions(-)
google-beta provider	View Diff	3 files changed, 13 insertions(+), 4 deletions(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
320	305	15	0

Affected Service Packages

• container

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