feat(venv): add secure per-launch find-links support

[m199369309]

Summary

• add a typed virtual_env_find_links launch parameter across REST, Supervisor, Worker, sync/async clients, and CLI
• validate request-level paths on each target Worker before invoking uv
• restrict paths to administrator-configured roots via XINFERENCE_VIRTUAL_ENV_FIND_LINKS_ALLOWED_ROOTS (default: ${XINFERENCE_HOME}/wheels)
• canonicalize paths, reject URLs/relative paths/files/symlink escapes, check readability, and deduplicate while preserving order
• preserve configured model/pip find-links and append validated per-launch directories
• document CLI and Python usage plus distributed-deployment and trust requirements

Dependency

Depends on #5386 so direct FlashInfer post-install commands honor the same configured find-links sources as the main virtualenv installation.

This PR is intentionally stacked on #5386. The shared commit will disappear from this PR after #5386 is merged.

Security

Validation happens on each selected Worker because the Supervisor cannot safely validate Worker-local filesystems. Allowed roots must be controlled by administrators and must not be writable by untrusted users, since installing wheels executes package code.

Set XINFERENCE_VIRTUAL_ENV_FIND_LINKS_ALLOWED_ROOTS to an empty value to disable request-level local paths.

Tests

• pytest -q xinference/core/tests/test_virtual_env_manager.py xinference/core/tests/test_worker.py xinference/core/tests/test_launch_strategy.py xinference/core/tests/test_model_replica_scale.py (163 passed)
• pre-commit run --files <modified files>
• CLI help smoke test for --virtual-env-find-link

[gemini-code-assist]

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