fix(agent): fall back to non-streaming on JSONDecodeError

[liuhao1024]

What does this PR do?

When using custom:llmgateway (or similar providers) with streaming enabled, tool-call turns can fail with json.JSONDecodeError: Expecting value: line 1 column 1 (char 0). The agent retries the same streaming path multiple times, then the tool workflow fails entirely.

Root Cause

The streaming error handling in _interruptible_streaming_api_call only recognizes "stream not supported" as a signal to fall back to non-streaming. When a provider returns malformed or empty SSE responses for tool-call turns (raising json.JSONDecodeError), the error is treated as a generic failure and retried with the same streaming path — which fails identically each time.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• See commit messages for detailed changes

How to Test

1. Run pytest tests/ -q — all tests should pass
2. Verify the specific scenario described above is resolved

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.4.1

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture and workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A

[teknium1]

Thanks for isolating the streaming failure mode. The underlying fallback is still absent on current main: run_agent.py:1093-1109 explicitly excludes JSONDecodeError from the malformed-stream classifier, while agent/chat_completion_helpers.py:2816-2865 only sets _disable_streaming for unsupported-stream or Bedrock-IAM errors.

Problems

• The production hunk is stale. run_agent.py:4764-4769 is now only a forwarder; commit 0430e71ec moved the streaming caller to agent/chat_completion_helpers.py. The current retry branch is at agent/chat_completion_helpers.py:2619-2865.
• The added test checks only the flag after a direct helper call. It does not verify the next loop iteration takes the non-streaming branch at agent/conversation_loop.py:1276-1316.
• The change makes any raw JSONDecodeError session-wide transport state. The linked [Bug]: streaming gets disabled for whole session on one error and causes errors, should be by request #25723 discussion flags session-wide disablement as a possible problem, so the intended scope needs an explicit decision and regression coverage.

Suggested changes

• Rebase the logic conceptually onto agent/chat_completion_helpers.py and test streaming-to-non-streaming routing through the conversation loop.
• Narrow and document the incompatibility signature and fallback lifetime.

Automated hermes-sweeper review.