Test Gap: No test to validate GCU apply-patch completes within expected time bounds

[rimunagala]

Is it platform specific

generic

Importance or Severity

Critical

Description of the test gap

GCU has no test that verifies apply-patch completes within a reasonable
time when processing config changes. GCU performance regressions — including
near-hung behavior — go completely undetected in CI.

Known repro:

• ACL table configured with ports bound to a large list of interfaces
• GCU patch replaces the ports field with a very short list
• GCU internal sort/diff logic hangs or takes 60+ minutes

Affected Area

GCU (Generic Config Updater) — tests/generic_config_updater/

Impact of missing tests and proposed tests

A build where GCU hangs passes all current tests because no test asserts
timing. The regression only surfaces in production.

Proposed fix: new test test_gcu_acl_ports_replace_large_to_small in
tests/generic_config_updater/test_gcu_acl_ports_replace.py that:

1. Creates ACL table bound to all available ports
2. Replaces ports field with 2 ports via GCU patch
3. Asserts patch completes within get_gcu_timeout(duthost)
   This test FAILS on a buggy build and PASSES after the fix.