Update: Harden middleware infra diagnostics and live smoke

[SI-RUI-ZHANG]

原问题

• PR Update: Harden middleware infra diagnostics and live smoke #13 became conflicting after main advanced with the controller and payload-normalization changes.
• Repo-local middleware status checks could false-green when another dev runtime was already listening on 127.0.0.1:8787.
• The generic ./scripts/smoke_e2e.sh --live path had regressed to a skip, so direct upstream traversal no longer had a maintained smoke path.
• Source-checkout editable installs still needed clearer guidance to prefer repo-local Python wrappers over brittle installed entrypoints.

本次修复

• Reapplied the infra hardening on top of current main without reverting the controller-oriented docs and support matrix.
• Added repo-local dev runtime identity stamping to /healthz plus explicit status outcomes for different or unknown runtimes on the same port.
• Restored direct upstream --live smoke, kept the supported controller-matrix smoke paths, and documented both in the contributor/operator docs.
• Added regression coverage for the dev runtime identity flow and health payload propagation.

Changed files list
CONTRIBUTING.md
README.md
modeio_middleware/dev_runtime.py
modeio_middleware/http_transport.py
scripts/dev_gateway.py
scripts/smoke_e2e.sh
tests/integration/test_gateway_contract.py
tests/unit/test_dev_runtime.py

Reviewer: @SI-RUI-ZHANG