Add comprehensive PVST test plan document

[devin-ai-integration]

Description of PR

Summary:
This PR adds a comprehensive test plan document for validating Per-VLAN Spanning Tree (PVST) functionality in SONiC. The test plan covers PVST and Rapid PVST (RPVST) protocol implementations using the T0 topology configurations with two VLAN and four VLAN setups.

The document includes:

• Overview of PVST protocol and variants (PVST/RPVST/MSTP)
• Test environment documentation for two_vlan_a and four_vlan_a topologies from ansible/vars/topo_t0-isolated-d16u16s1.yml
• 21 comprehensive test cases covering per-VLAN STP instance independence, interface-specific configuration, topology change resilience, protocol variant testing, and configuration persistence
• API reference for functions in spytest/apis/switching/pvst.py
• Validation criteria and pass/fail definitions

Reviewer focus areas:

• Verify API function names/parameters match actual implementation in spytest/apis/switching/pvst.py
• Verify VLAN topology configurations match ansible/vars/topo_t0-isolated-d16u16s1.yml
• Check test case coverage is sufficient for PVST validation

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?

To provide a structured test plan for validating PVST functionality in SONiC, enabling systematic testing of per-VLAN spanning tree instances using the existing SPyTest PVST API.

How did you do it?

Created a comprehensive markdown document that:

1. Documents the test environment using existing T0 topology configurations
2. Defines 21 test cases organized by functional area
3. References the existing PVST API functions from spytest/apis/switching/pvst.py
4. Establishes clear pass/fail validation criteria

How did you verify/test it?

This is a documentation-only change. The test plan document was reviewed for:

• Accuracy of API function references against spytest/apis/switching/pvst.py
• Accuracy of topology configurations against ansible/vars/topo_t0-isolated-d16u16s1.yml
• Completeness of test coverage for PVST functionality

Any platform specific information?

N/A - This is a test plan document applicable to all platforms supporting PVST.

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

The test plan targets T0 topology with two_vlan_a and four_vlan_a configurations.

Documentation

This PR itself is documentation - a test plan for PVST functionality.

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Link to Devin run: https://cisco-demo.devinenterprise.com/sessions/bdf0af5058584cdfa8d54c0d72cd2882
Requested by: Arthur Poon (arthur.poon@cognition.ai) / @arthurkkp-cog

[devin-ai-integration]

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