ComputeRoute: Fix network and next_hop_gateway diff discrepancies between tf and direct controllers #13581

Bug Description

In the 1.157 preview evaluation comparing the tf (default) and direct controllers for compute.cnrm.cloud.google.com/ComputeRoute, all 51 evaluated instances (100%) report no diff_fields under the tf controller, but report diff_fields (network, next_hop_gateway) under the direct controller (currently marked RECONCILE_STATUS_HEALTHY because ComputeRoute does not support in-place updates in GCP, so the diff is ignored/allowed during reconcile, but a field-level diff still exists).

Preview Summary (result_1.157)

Discrepant diff_fields in direct Controller

Observed Diff Combinations (tf vs. direct)

Workflow & Instructions

Maintain a journal file (e.g., journal_computeroute.md) throughout the entire effort to track all replication attempts, test runs, newly discovered field discrepancies, and fixes.

Step 1: Replication of the Bugs

1. Run Existing Tests:
   • Run TestMigrationToDirect for the existing tests for ComputeRoute.
2. Verify Replication in Logs:
   • Check the _migration_diffs.json file of each test. The issue is replicated once _migration_diffs.json has an entry for "controller": "direct", and the fields that have diffs (network, next_hop_gateway) show up in the entry.
   • If the resource's direct controller supports updates in GCP, the Phase 3 log _http_migration_phase3_direct_takeover.log should also contain a non-GET call.
3. Create New Tests (If Needed):
   • If the existing tests cannot reproduce the issue, create new tests covering network and nextHopGateway and run TestMigrationToDirect with real GCP to reproduce the problem.
4. Commit or Stop:
   • If the problem is successfully reproduced: Create a commit with the reproduction test(s), test logs, and journal file, then proceed to Step 2.
   • If the problem cannot be reproduced: Commit the test logs and journal file, comment on the PR that the issue cannot be reproduced, stop, and create the PR.

Step 2: Fix the Discrepancies and Verify

1. Implement the Fix:
   • Once the problem is successfully reproduced, create a fix so the direct controller and tf controller no longer have a logic/normalization discrepancy for network and next_hop_gateway (e.g., full URL vs. partial selfLink / relative resource name normalization).
2. Handle Additional Field Discrepancies:
   • If you find other fields with discrepancies during testing, create a test case that reproduces each newly discovered issue in a separate commit, and fix them.
3. Re-run and Verify Clean Migration Logs:
   • Once all discrepancies are discovered and fixed, re-run TestMigrationToDirect and keep the new test logs, which must show:
     • No migration diff in _migration_diffs.json.
     • No non-GET call in _http_migration_phase3_direct_takeover.log during Phase 3.
4. Create Separate Fix PR:
   • Put the controller fixes, the updated clean test logs, and the journal file in a separate PR.