fix(docker): use host-gateway callbacks on macOS

[TaylorMutch]

Summary

Fix macOS Docker-backed gateway startup by using Docker's host-gateway callback route instead of trying to bind the Docker VM bridge gateway IP.

Related Issue

Related to #1307

Changes

• Default Docker gateway callback routing to host-gateway on macOS unless host_gateway_ip is explicitly configured.
• Keep native Linux Docker bridge listener behavior and explicit host_gateway_ip overrides.
• Update Docker driver tests and docs for macOS/Docker Desktop callback routing.

Testing

• cargo test -p openshell-driver-docker docker_gateway_route
• cargo check -p openshell-server --bin openshell-gateway
• Local Docker-backed gateway smoke: started patched gateway on 127.0.0.1:17670, registered mTLS CLI gateway, created macos-local-test, ran echo openshell-sandbox-ok, and verified cleanup
• mise run pre-commit passes
  • Fails in an existing macOS trust-store-dependent CLI unit test: run::tests::http_health_check_supports_plain_http_endpoints
• E2E tests added/updated
  • Requested with the test:e2e label.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[github-actions]

Label test:e2e applied for c0a9306. Open Branch E2E Checks, find the run for commit c0a9306, and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.