[Cisco] Enable unresolved-route class ID before Copp warmboot replay.

[ram316583]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

AgentCoppTest::UnresolvedRouteNextHopToLowPriQueue was setting FLAGS_classid_for_unresolved_routes inside the per-test setup() lambda.

On warmboot, setup() is skipped. SAI route replay then programs unresolved CPU routes with Metadata 0, so traffic hits IP2ME (CPU queue 2) instead of the unresolved-route ACL (CPU queue 0) and the test fails.

Set FLAGS_classid_for_unresolved_routes = true in setCmdLineFlagOverrides() so the flag is present before agent init / SAI replay on both coldboot and warmboot. This also drops the Chenab vendor special-case that left the flag off in setup(). The flag is now always enabled for this test fixture.

Test Plan

[result] AgentCoppTest/0.UnresolvedRouteNextHopToLowPriQueue [coldboot]: PASSED (exit code 0)
[result] AgentCoppTest/0.UnresolvedRouteNextHopToLowPriQueue [warmboot]: PASSED (exit code 0)
[result] AgentCoppTest/1.UnresolvedRouteNextHopToLowPriQueue [coldboot]: PASSED (exit code 0)
[result] AgentCoppTest/1.UnresolvedRouteNextHopToLowPriQueue [warmboot]: PASSED (exit code 0)

[srikrishnagopu]

Closing this based on the discussion

cc: @jasmeetbagga @vasantspatil