feat: surface swallowed response_future errors in interception proxy

[rasdani]

Summary

Companion to #1194 (SSE keepalive). Fixes two remaining error-swallow paths in interception_utils.py analogous to the chunk-write failure path that #1191 already addressed.

Both paths currently caught exceptions from await response_future and silently let the handler return. On the streaming side, the chunk loop had already written data: [DONE] before the await, so the agent sees a clean "empty completion" → exits 0 → silent empty trajectory. On the non-streaming side, the handler returned HTTP 500 to the agent but never set state["error"], so rubrics/metrics couldn't see the failure.

Changes

• Streaming path (_handle_streaming_response post-loop): narrow except BaseException → except Exception with explicit except asyncio.CancelledError: raise (normal teardown via unregister_rollout must not be misclassified as error). Set state["error"] = StreamInterrupted(...) via _set_rollout_error.
• Non-streaming path (_handle_request else branch): set state["error"] = InterceptionError(...) before returning HTTP 500.
• Add InterceptionError class alongside StreamInterrupted so rubrics/metrics can distinguish the two shapes (streaming cut → truncated SSE body; non-streaming failure → HTTP 500 to agent's OpenAI client).
• Keep log levels at debug — messages may contain upstream internals (URLs with tokens, traceback fragments); ERROR severity would ship those to centralized log aggregation unnecessarily.

Tests

Adds two tests in tests/test_interception_utils.py:

• test_streaming_response_future_failure_surfaces_to_state
• test_non_streaming_response_future_failure_surfaces_to_state

7 tests total pass locally. Ruff + format clean.

Independent of #1194

This branch is off main and does not depend on the keepalive PR. Both can merge in either order.

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Note

Medium Risk
Changes error propagation in the interception proxy for both streaming and non-streaming requests, which can alter rollout termination behavior and metrics capture. Risk is moderate because it touches async control flow and cancellation handling but is covered by new tests.

Overview
Ensures failures from response_future are no longer silently swallowed in the interception proxy, so rollouts halt visibly instead of producing empty/clean-looking completions.

For non-streaming requests, exceptions while awaiting the intercepted model result now set state["error"] (via a new InterceptionError) before returning HTTP 500. For streaming requests, exceptions from the post-stream response_future await now set state["error"] as StreamInterrupted, while CancelledError is explicitly re-raised to avoid misclassifying normal teardown.

Adds tests covering both streaming and non-streaming response_future failure paths.

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[mikasenghaas]

lgtm