gateway_proxy: shorten upstream read timeout and surface stream interruptions

[rohita5l]

What

Two changes to the ucode loopback refresh proxy (src/ucode/gateway_proxy.py) so a stalled/interrupted upstream stream degrades cleanly instead of hanging for 10 minutes or rendering as a silent truncation.

B1 — lower the upstream read timeout 600s → 120s. On the relayed path, the pings the proxy sees are AIGW's keep-alive frames (~10s) that cover only healthy streams; AIGW aborts a genuine stall at ~60s and closes cleanly (proxy sees EOF and relays it). The proxy read timeout then only needs to catch a dark upstream — pod crash / network partition, where neither tokens nor pings arrive — and 120s catches that in ~2 min instead of 10. It stays well above the 10s keep-alive interval so slow-but-healthy streams never trip it. connect/write/pool unchanged.

B2 — emit a terminal SSE error frame on mid-stream drop. When iter_raw raises httpx.HTTPError after the head is already sent, write a well-formed Anthropic event: error frame before closing so Claude Code retries cleanly rather than rendering "incomplete".

B3 — corrected the timeout-rationale comment to reflect the relayed path (AIGW keep-alive frames, not Anthropic's own pings).

Correctness guard on B2

_relay_response handles every response and is byte-transparent over Content-Encoding. So the error frame is gated on the body being uncompressed text/event-stream — injecting plaintext SSE framing into a partial gzip body or a non-streaming JSON body would corrupt it, which is strictly worse than a clean truncation. The frame also leads with a double CRLF to force an SSE event boundary in case the drop landed mid-line.

Caveat — this does not fully fix the customer hang

With AIGW keep-alive on, each : ping resets httpx's read timer, so B1 only shortens the dark-upstream case. A keep-alive-fed stall is still bounded only by AIGW's ~600s webClientRequestTimeoutMs until #2390858 (A1) lands. B1/B2 are necessary but not sufficient; A1 remains the primary lever.

Testing

• uv run ruff check . — clean
• uv run ruff format --check — clean
• uv run pytest tests/test_gateway_proxy.py — 21 passed

This pull request and its description were written by Isaac.