[docs/testplan][bgp]: Add test plan for PrefixListMgr refactor and SUPPRESS_PREFIX

[nanali-msft]

Description of PR

Summary:
Add a new test plan document docs/testplan/PrefixListMgr-Refactor-Test-Plan.md
that defines regression and feature coverage for sonic-net/sonic-buildimage#26937
("[PrefixListMgr]: Refactor PrefixListMgr to support multiple prefix types via
config registry").

The plan covers:

• ANCHOR_PREFIX regression on UpstreamLC / UpperSpineRouter (CLI round-trip,
  re-run of existing tests/bgp/test_prefix_list.py, rejection on non-spine,
  persistence across config reload and docker restart bgp, and validation
  that PrefixListMgr is now registered on every device).
• New SUPPRESS_PREFIX type end-to-end on physical testbeds: CLI add/remove/
  status for IPv4 + IPv6, FRR show <ip|ipv6> prefix-list rendering,
  any-device acceptance, constants.yml name override path, registry-default
  fallback, and persistence.
• Negative / validation cases: unsupported prefix type via CLI and via direct
  CONFIG_DB write, malformed prefixes, status allowed on every device, and
  unchanged chassis-supervisor skip behavior.

Each test case includes topology, step-by-step procedure, expected results,
pass criteria, and Python skeletons (helpers, fixtures, loganalyzer hookup,
constants.yml override patcher) that map 1:1 to a follow-up
tests/bgp/test_prefix_list_suppress.py implementation. Pytest invocation
commands for physical (t0/t1/t2) and VS testbeds are included.

Fixes # (n/a — doc-only follow-up for sonic-net/sonic-buildimage#26937)

Type of change

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

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Approach

What is the motivation for this PR?

sonic-net/sonic-buildimage#26937 refactors PrefixListMgr to be data-driven
via a per-prefix-type registry and introduces a new generic SUPPRESS_PREFIX
type. The PR also changes bgpcfgd to register PrefixListMgr
unconditionally and relaxes prefix_list status to be allowed on every
device. We need an explicit test plan to (1) prove no regression for the
existing ANCHOR_PREFIX flow on UpstreamLC / UpperSpineRouter and (2) verify
the new SUPPRESS_PREFIX feature, constants.yml override path, and per-type
device gating on physical testbeds.

How did you do it?

Authored docs/testplan/PrefixListMgr-Refactor-Test-Plan.md with:

• Scope, PR change summary, risk areas, and testbed coverage matrix.
• 16 test cases grouped into ANCHOR_PREFIX regression (TC-A1..A5),
  SUPPRESS_PREFIX (TC-S1..S6), and negative/validation (TC-N1..N5).
• A detailed implementation guide with reusable helpers
  (op_prefix_with_cmd, verify_prefix_list_in_db,
  verify_frr_prefix_list_entry, get_suppress_pl_names), a function-scoped
  cleanup fixture, and a LogAnalyzer fixture so intentional log_warn
  lines for negative cases are expected and any new errors are caught.
• Concrete pytest invocation commands for physical and VS testbeds.

How did you verify/test it?

Doc-only change; verified internal links and the existing baseline test
(tests/bgp/test_prefix_list.py) referenced by the plan exist in the repo.
The implementation (tests/bgp/test_prefix_list_suppress.py) will be
submitted in a follow-up PR and validated on physical t0/t1/t2 testbeds per
the plan.

Any platform specific information?

None. ANCHOR_PREFIX cases require a chassis with an UpstreamLC line card
(t2). SUPPRESS_PREFIX and negative cases run on any topology (t0/t1/t2/vs).

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

• ANCHOR_PREFIX regression: t2 (UpstreamLC / UpperSpineRouter).
• SUPPRESS_PREFIX + negative cases: t0, t1, t2.
• VS (vms-kvm-t0): smoke-test for CLI plumbing and bgpcfgd startup only.

Documentation

docs/testplan/PrefixListMgr-Refactor-Test-Plan.md

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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