[ExternalTest](feat) Add pluggable external operator repo test framework

[TecJesh]

Summary

Add a pluggable external operator repo test framework. Users specify external
GitHub repositories and their test cases in a YAML config file. The workflow
automatically clones each repo, runs tests sequentially per repo, triggers AI
fix on failure, then proceeds to the next repo.

Core Design

Aspect	Description
Config-driven	Single YAML file (external_test_config.yaml), repos grouped with their test cases
Master switch	enabled field (default false), three-tier priority: CLI > env var > YAML
Three modes	inline (blocks main flow), standalone (non-blocking), off (skipped)
Execution order	Strictly serial across repos (first-configured-first-run), per-repo parallel via pytest -n 8
AI fix	AI fix + retest loop on failure, up to max_retries attempts, changes gated to the external repo directory

Changes

src/TA_main2main_workflow/
├── external_test/                              # [new]
│   ├── __init__.py
│   ├── external_test_config.yaml               # default config template
│   ├── config_loader.py                        # YAML loading + env override
│   └── runner.py                               # clone → install → pytest → fix
├── utils/
│   ├── config.py                               # [modified] +3 fields
│   └── context.py                              # [modified] +2 fields
└── flow.py                                     # [modified] external test stage integration

New Environment Variables

Variable	Default	Description
TA_EXTERNAL_TEST_ENABLED	false	Master on/off switch
TA_EXTERNAL_TEST_CONFIG	Built-in default path	Config file path
TA_EXTERNAL_TEST_MODE	inline	inline / standalone / off
TA_EXTERNAL_TEST_PROCS	8	pytest parallel workers per repo
TA_EXTERNAL_TEST_MAX_RETRIES	5	AI fix retry limit per repo
TA_EXTERNAL_TEST_TIMEOUT	7200	Per-repo test timeout (seconds)

• run pre-commit run --all-files to ensure code style
• unit tests added or updated (if applicable)