fix(server): use internal endpoint resolution for proxy route

[suger-m]

Fixes #402

Summary

This PR fixes proxy endpoint resolution in proxy_sandbox_endpoint_request.

Previously, the proxy route called get_endpoint(sandbox_id, port), which may return a non-internal/proxied endpoint depending on runtime configuration. That behavior is incorrect for server-side request forwarding.

This change updates the proxy route to explicitly call get_endpoint(sandbox_id, port, resolve_internal=True), ensuring requests are always forwarded to the sandbox internal endpoint.

Tests are updated accordingly:

• proxy route stubs now accept the resolve_internal argument
• an assertion is added to verify resolve_internal=True is passed in the proxy path

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Executed:

• uv run pytest tests/test_routes_proxy.py
• uv run pytest tests/test_routes_endpoint_behavior.py

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[Pangjiping]

LGTM