feat(proxy): raw-passthrough /v1/chat/completions — fixes Codex / OpenAI tool calls

[KillerQueen-Z]

Problem

/v1/chat/completions still went through the SDK's typed chat_completion_stream, which crashes on streamed tool calls:

File ".../blockrun_llm/solana_client.py", _aiter_and_archive
    if choice.delta.content:
AttributeError: 'dict' object has no attribute 'delta'

Root cause (confirmed reproduced on Solana with a real model):

1. The SDK's ToolCall schema requires id / function.name / arguments, but streaming tool-call argument-fragment frames (id/name absent, partial args) don't satisfy it.
2. _aiter_sse_chunks therefore falls back to ChatCompletionChunk.model_construct(...), which doesn't parse nested models → choices stay as raw dicts.
3. _aiter_and_archive then does choice.delta.content on a dict → crash.

Plain chat works (text frames validate fine); any tool call breaks. This hits every OpenAI client doing tool use — notably Codex with wire_api=chat.

Fix — extend the verbatim passthrough to /v1/chat/completions

#6 made /v1/messages a byte-for-byte x402-signed passthrough. This generalizes that helper (_forward_anthropic → _forward_passthrough, taking a headers arg) and routes /v1/chat/completions through it too.

• Body forwarded byte-for-byte, only the x402 signature added → the SDK's streaming/parsing/archiving is no longer in the hot path, so the crash can't be reached and streamed tool_calls survive intact.
• Reuses the merged Anthropic path's semaphore gating + real-upstream-status handling (no more unconditional 200 on upstream errors).
• One sidecar now serves Claude Code (/v1/messages), Codex / OpenAI clients (/v1/chat/completions), all as pure signing passthroughs.

This is the structural direction: the proxy is a thin x402 signer; the gateway owns the protocol. No edge translation, so SDK streaming bugs can't surface in the proxy.

Verification (real, end-to-end)

On Solana (sol.blockrun.ai) with real paid models:

Endpoint	Before	After
/v1/chat/completions stream + tools	❌ 'dict' object has no attribute 'delta'	✅ tool_calls + full args {"city":"Madrid"} + finish_reason=tool_calls
/v1/messages stream + tools	✅	✅ (unchanged)
non-stream chat	✅	✅

pytest: 84 passed (1 pre-existing litellm-version canary deselected). Compile clean.

Note on /v1/responses (Codex Responses API)

The gateway has no native /v1/responses, so it can't be a verbatim passthrough; the existing bridge stays as-is. Recommended Codex config is wire_api = "chat" → routes through this fixed /v1/chat/completions. A native gateway /v1/responses would be the clean long-term answer for wire_api=responses.