[codex] Keep Vertex streams open during idle periods

[prasadus92]

Summary

Fixes the local Vertex proxy streaming path so long Vertex responses do not get cut off during idle periods.

Changes:

• adds a separate no-read-timeout HTTPX timeout for streaming requests
• returns structured SSE error events for upstream stream failures
• drains upstream HTTP error bodies instead of raising inside the generator
• documents the incomplete-chunked-read troubleshooting path
• adds regression coverage for streaming read timeouts

Why

Hermes saw RemoteProtocolError / incomplete chunked reads when the proxy hit an upstream httpx.ReadTimeout mid-stream. That closed the response abruptly and made the client see a broken chunked response instead of a clean model error.

Validation

• .venv/bin/pytest -q -> 20 passed
• .venv/bin/ruff check .
• git diff --check
• curl -fsS http://127.0.0.1:8787/health -> healthy against project luminik-459117

Operational note

The local launchd service was restarted and is currently healthy on 127.0.0.1:8787.