[Architecture] Centralize PyATS common utilities into dedicated external repository

[aitestino]

Based on a conversation with @oboehmer today.

Summary

Evaluate and plan the creation of a centralized repository (e.g., NAC-TEST-PyATS) to host shared PyATS utilities that are currently duplicated across architecture-specific repositories.

Problem Statement

Currently, each NAC architecture repository (nac-sdwan-terraform, nac-aci, nac-catalyst-center, etc.) maintains its own pyats_common/ directory containing base test classes and utilities. This creates several challenges:

1. Code Duplication - Similar logic is repeated across multiple repositories
2. Maintenance Burden - Bug fixes and improvements must be applied to each repo individually
3. Implementation Drift - Implementations can diverge over time without a single source of truth
4. CI/CD Complexity - Testing shared patterns requires running tests across multiple repos
5. Scalability Concerns - As new architectures are added, the duplication problem compounds

Proposed Solution

Create a centralized external repository (working name: NAC-TEST-PyATS) within the Net-as-Code organization that hosts shared PyATS utilities.

Proposed Layer Structure

Layer	Location	Contents
Core Framework	nac-test	Orchestrator, reporting, SSHTestBase, NACTestBase, connection broker
PyATS Common	NAC-TEST-PyATS (proposed)	Shared device resolver patterns, common verification utilities, reusable test helpers
Architecture-Specific	nac-sdwan-terraform, nac-aci, etc.	Data model parsing specific to that architecture's schema structure

Benefits

• Single source of truth for shared PyATS code
• Centralized CI/CD - comprehensive testing in one location
• Versioned releases - architecture repos can pin to stable versions
• Easier onboarding - new architectures have a clear starting point and patterns to follow
• Reduced maintenance - fixes applied once, benefit all architectures

Considerations

• Dependency management for architecture repos
• Release coordination for breaking changes
• Clear guidelines on what belongs in common vs. architecture-specific repos

Priority

Medium - Not blocking current work, but important for long-term maintainability as the number of supported architectures grows.