[route] Fix route consistency snapshot flake

[LinJin23]

Description of PR

Summary:
Serialize ASIC_DB route snapshot collection on the controller to avoid flaky failures from concurrent pytest-ansible calls.

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO): https://msazure.visualstudio.com/One/_workitems/edit/38896300/?view=edit
Failure type: other - test infrastructure race condition

Tested branch

• master
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• N/A

Test result

• Reproduction before fix: SONiC.20251110.42 - Attempts 0 and 3 failed with route snapshot timeouts: https://elastictest.org/scheduler/testplan/6a7d796279a16f6b66031ae6
• Validation after fix: _SONiC.20251110.48 - All 4 test cases passed: https://elastictest.org/scheduler/testplan/6a9f9f2eef719d7dfd657106

Approach

What is the motivation for this PR?

get_route_prefix_snapshot_from_asicdb launches concurrent controller threads that reuse the same pytest-ansible/SonicHost dispatcher. In the observed failures, one Ansible transaction raised AnsibleConnectionFailure and did not post its completion signal, causing the main thread to report a route snapshot timeout.

How did you do it?

Removed the controller-side threading and queue logic and collected each ASIC snapshot serially.

How did you verify/test it?

Ran route/test_route_consistency.py on 202511; all 4 test cases passed.

Any platform specific information?

The failure was observed on an Arista-7280DR3AM-36 multi-ASIC HWSKU. The change is not platform-specific.

Supported testbed topology if it's a new test case?

N/A

Documentation

N/A

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[mssonicbld]

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