feat: add assertoor plugin for testnet testing access

[reldothescribe]

This PR adds an Assertoor plugin following the Dora plugin pattern.

Features Exposed

• List running tests - Query available test definitions
• Get test status/results - Retrieve test run status and detailed results
• Get task logs - Fetch task logs with full details
• Deep links to test pages - Generate web UI links for tests and tasks

API Functions

Test Management

• get_tests(network) - List available test definitions
• get_test_runs(network, test_id=None, limit=100) - List test runs
• get_test_run_details(network, run_id) - Get detailed test run with tasks
• get_test_run_status(network, run_id) - Get test run status summary

Task Inspection

• get_task_details(network, run_id, task_index) - Get task details with logs

Deep Links

• link_test_run(network, run_id) - Link to test run page
• link_task(network, run_id, task_index) - Link to task page
• link_task_logs(network, run_id, task_index) - Link to task logs

Implementation

• Created plugins/assertoor/ with config.go, plugin.go, examples.go, examples.yaml
• Created plugins/assertoor/python/assertoor.py - Python module using httpx
• Registered plugin in pkg/server/builder.go
• Updated sandbox init.py to include assertoor module
• Copied Python module to sandbox directory

Assertoor URLs are discovered from cartographoor (similar to Dora) via ServiceURLs.Assertoor.