fix(source/cloud-sql-admin): validate source instance for backup-run restore

[anxkhn]

Description

Source.RestoreBackup in internal/sources/cloudsqladmin/cloud_sql_admin.go validates the wrong field on the BackupRun-ID restore path. When backup_id is a numeric BackupRun ID, the code populates RestoreBackupContext from sourceProject and sourceInstance, but the guard checks sourceProject and targetInstance:

if backupRunID, err := strconv.ParseInt(backupID, 10, 64); err == nil {
    if sourceProject == "" || targetInstance == "" { // checks targetInstance
        return nil, fmt.Errorf("source project and instance are required when restoring via backup ID")
    }
    request.RestoreBackupContext = &sqladmin.RestoreBackupContext{
        Project:     sourceProject,
        InstanceId:  sourceInstance, // sourceInstance is what's actually consumed
        BackupRunId: backupRunID,
    }
}

targetInstance is already mandatory for the API call itself (service.Instances.RestoreBackup(targetProject, targetInstance, request)), so the guard never adds anything for it. The only thing it lets through is an empty source_instance, which then sends a malformed RestoreBackupContext{InstanceId: ""} to the API.

The error message already reads "source project and instance are required", and both the tool parameter description (internal/tools/cloudsql/cloudsqlrestorebackup/cloudsqlrestorebackup.go) and the docs (docs/en/integrations/cloud-sql-admin/tools/cloudsqlrestorebackup.md) state that both source_project and source_instance are required when backup_id is a BackupRun ID. So this is a one-character copy/paste bug, not intended behavior.

Fix: guard on sourceInstance == "" instead of targetInstance == "". The error string is unchanged. A regression row is added to the existing tests/cloudsql/cloud_sql_restore_backup_test.go table for the case where source_project is set but source_instance is omitted, which fails before the fix (an empty InstanceId reaches the API) and passes after.

PR Checklist

• Make sure you reviewed
  CONTRIBUTING.md
• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
• Make sure to add ! if this involve a breaking change

No tracking issue was opened: this is a small, self-contained correctness fix with a regression test, and the open type: bug issues are already assigned. Happy to file one and link it with Fixes #<n> 🦕 if you would prefer that first. The docs and the tool parameter description already mark both source fields as required, so no docs change is needed; this is not a breaking change (validation is only tightened for input that would already have failed at the API).

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[gemini-code-assist]

Code Review

This pull request corrects a validation check in the Cloud SQL Admin source's RestoreBackup function, ensuring that sourceInstance is verified instead of targetInstance when restoring via a backup ID. A new integration test case has been added to cover this scenario. I have no feedback to provide as there are no review comments.