feat: allow host.docker.internal for local inference

[EdwallScaffold]

Summary

This PR fixes the issue where host.docker.internal was being blocked by the sandbox proxy, preventing local inference scenarios (vLLM, Ollama) from working within OpenShell sandboxes.

Problem

When users configured local inference endpoints using host.docker.internal:8000, the sandbox proxy would reject the connection. This broke legitimate use cases like running vLLM or Ollama on the host machine.

Solution

Added a special case in resolve_and_reject_internal() to allow host.docker.internal to resolve to private IP addresses, while still blocking loopback and link-local addresses for security.

Changes

• Modified resolve_and_reject_internal() function
• Added special handling for host.docker.internal
• Still blocks loopback (127.x.x.x) and link-local (169.254.x.x)
• Added 3 unit tests
• Improved error messages with helpful hints

Testing

• All existing tests pass (74 tests in proxy module)
• New tests verify Docker bridge IP ranges
• End-to-end test with vLLM on host: SUCCESS

Security

This change maintains security boundaries:

• Loopback addresses (127.0.0.0/8) are still blocked
• Link-local addresses (169.254.0.0/16) are still blocked
• Only host.docker.internal gets special treatment

Related Issues

Fixes: #263
Related: NVIDIA/NemoClaw#314
Related: NVIDIA/NemoClaw#385

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