discoveryengine: add pre-delete hook to detach stores on destroy

[btkelly]

This PR adds a custom pre_delete hook for the google_discovery_engine_data_connector resource to handle destruction order dependencies. It introduces a virtual field detach_stores_on_destroy that, when set to true, detaches linked data stores from search engines before deletion, preventing dependency errors.

Manual tests were successful on a failing module. A test case was added but skipped to avoid failing builds due to IdP setup requirements (matching the pattern for this resource).

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discoveryengine: added `detach_stores_on_destroy` field to `google_discovery_engine_data_connector` resource to handle destruction order dependencies

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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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 ( 4 files changed, 253 insertions(+))
google-beta provider: Diff ( 4 files changed, 253 insertions(+))

[modular-magician]

Non-exercised tests

🔴 Tests were added that are skipped in VCR:

• TestAccDiscoveryEngineDataConnector_detachStoresOnDestroy

Tests analytics

Total tests: 41
Passed tests: 35
Skipped tests: 5
Affected tests: 1

Click here to see the affected service packages

• discoveryengine

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

• TestAccDiscoveryEngineDataStore_discoveryengineDatastoreKmsKeyNameExample

Get to know how VCR tests work

[modular-magician]

🔴 Tests failed during RECORDING mode:
TestAccDiscoveryEngineDataStore_discoveryengineDatastoreKmsKeyNameExample [Error message] [Debug log]

🔴 Errors occurred during RECORDING mode. Please fix them to complete your PR.

View the build log or the debug log for each test