[sandbox] feat: discover external provider modules

[tongyx361]

Summary

External sandbox providers currently require modifying the built-in provider table. Add module discovery through UNI_AGENT_SANDBOX_PLUGINS so separately installed providers can register themselves.

Changes

• Load comma-separated module names when resolving an external provider.
• Reserve built-in names and preserve import failures across retries, cleaning stale registrations from failed imports.
• Document worker configuration and test discovery, retry behavior, and built-in name protection.

Validation

• PYTHONPATH=.:verl uv run --no-project --python /usr/bin/python python -m pytest -q tests/uni_agent/sandbox/test_sandbox_provider_discovery_on_cpu.py tests/uni_agent/sandbox/test_exec_error_policy.py: 34 passed. Tests used the runtime interpreter with Uni-Agent and the pinned public verl gitlink explicitly selected.
• pre-commit run --all-files --show-diff-on-failure: passed.

Internal validation and benefit

The integration use case is a separately installed sandbox provider that can be selected without adding provider code or dependencies to the built-in registry. No isolated internal latency, throughput or capacity benefit has been measured for plugin discovery itself. Validation for this PR is the discovery, import-failure/retry and built-in-name protection coverage listed above; no internal provider implementation or infrastructure is included.

Compatibility

Built-in providers keep lazy loading. External modules must be installed and configured on each worker; no migration is needed for existing configurations. Provider names reserved by built-in modules cannot be replaced. This is a standalone extension without a linked issue.

Checklist

• The PR is focused and explains why no issue is needed.
• The title follows the owning-layer format.
• Behavior is covered by tests; skipped validation is stated above.
• User-facing configuration and behavior changes are documented where applicable.
• Compatibility and migration requirements are documented.
• Logs, fixtures, and examples contain no credentials or private data.
• pre-commit run --all-files --show-diff-on-failure passes.