fix: compute instance internal ipv6 prefix length is not set in creation and read in syncing

[jingjia-peng]

Fixes hashicorp/terraform-provider-google#21520.

This ensures that Terraform correctly reads the compute_instance.network_interface.internal_ipv6_prefix_length from GCP and stores it in terraform.state. Previously, the absence of this value in terraform.state caused compute_instance to be considered modified, even when no changes were made to the configuration.

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compute: fix `compute_instance.network_interface.internal_ipv6_prefix_length` not being set or read in Terraform state  

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[roaks3]

Thanks for the fix. It looks like at least part of this was added with #8432, and for some reason this field was left out of the flattener/expander. It seems like there may have even been cases where the value of this field wasn't being submitted to the server as expected.

Will run tests and approve once they pass.

[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 ( 2 files changed, 88 insertions(+), 22 deletions(-))
google-beta provider: Diff ( 2 files changed, 88 insertions(+), 22 deletions(-))
terraform-google-conversion: Diff ( 2 files changed, 48 insertions(+), 44 deletions(-))

[modular-magician]

Tests analytics

Total tests: 1121
Passed tests: 1043
Skipped tests: 77
Affected tests: 1

Click here to see the affected service packages

• compute

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

• TestAccComputeInstance_internalIPv6PrefixLength

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[modular-magician]

🟢 Tests passed during RECORDING mode:
TestAccComputeInstance_internalIPv6PrefixLength [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