fix(agent): shape partial-stream stub to match active api_mode

[liuhao1024]

What does this PR do?

Fixes the partial-stream recovery stub for anthropic_messages mode. When a malformed Anthropic stream (non-contiguous content_block indices) crashes stream.get_final_message() with IndexError, the recovery stub now returns an Anthropic-shaped response (content list) instead of an OpenAI-shaped one (choices list). Previously the stub always used OpenAI shape, which AnthropicTransport.validate_response() rejected, causing 10 useless retries in the conversation loop.

Related Issue

Fixes #45908

Type of Change

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

Changes Made

• agent/chat_completion_helpers.py: In the partial-stream stub creation path, check agent.api_mode. When "anthropic_messages", return an Anthropic-shaped stub with content=[SimpleNamespace(type="text", text=...)] and stop_reason="max_tokens" instead of the OpenAI-shaped choices=[...] stub. This ensures AnthropicTransport.validate_response() accepts the stub on the first try.
• run_agent.py: Extend _is_provider_stream_parse_error() to also classify IndexError as a stream parse error for anthropic_messages mode. Non-contiguous content_block indices from upstream proxies cause the SDK accumulator to raise IndexError — this is an upstream protocol violation that should follow the same retry path as malformed event-stream frames.
• tests/agent/test_anthropic_stream_stub_shape.py: 8 regression tests covering: Anthropic stub has content list, passes validate_response, normalizes correctly, chat_completions stub unchanged, IndexError classified as parse error, mode-gated behavior, existing ValueError classification preserved.

How to Test

1. pytest tests/agent/test_anthropic_stream_stub_shape.py -xvs — all 8 tests pass
2. pytest tests/run_agent/test_partial_stream_finish_reason.py -xvs — all 9 existing tests pass (no regression)
3. To manually verify: configure an Anthropic-compatible proxy that sends non-contiguous content_block indices, send a message that triggers multi-block streaming (thinking + text), observe that the agent recovers gracefully instead of retrying 10 times with "response.content invalid" errors.

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

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 or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: agent/chat_completion_helpers.py::interruptible_streaming_api_call (stub creation path), run_agent.py::_is_provider_stream_parse_error (error classification)
• Blast radius: LOW — changes are in error recovery paths only; normal streaming is unaffected
• Related patterns: PARTIAL_STREAM_STUB_ID (stub identification), AnthropicTransport.validate_response() (validator), _STOP_REASON_MAP (stop_reason → finish_reason mapping)

[alt-glitch]

Duplicate of #45919 — same fix for #45908 (Anthropic-shaped partial-stream recovery stub in anthropic_messages mode, agent/chat_completion_helpers.py). #45919 is the earliest open PR in the cluster (#45920 closed, #45922 also open).

[liuhao1024]

Closing as duplicate of #45919 by @LeonSGP43, which implements the same fix for #45908 (Anthropic-shaped partial-stream recovery stub). That PR extracts a cleaner helper function with complete field coverage. It was opened earlier today and has passing CI.