feat(openrouter): native content-block streaming events

[LennyMalcolm0]

Stacked on #14 (core BlockStreamTracker + Anthropic), which it branches from. Depends only on the core primitive — sibling of the OpenAI chain. Retargets to master once #14 lands.

Gives ChatOpenRouter native _stream_chat_model_events / _astream_chat_model_events hooks so stream_events(version="v3") maps OpenRouter's stream directly to content-block events.

How it works

A bespoke converter (convert_openrouter_stream / async twin), sibling of the groq converter, builds text, reasoning, and tool-call blocks directly from OpenRouter's raw OpenAI-shaped delta, feeding the shared BlockStreamTracker. Tool-call args stream incrementally and finalize to a parsed dict. It does not depend on langchain-openai — it reuses only OpenRouter's own _create_usage_metadata.

OpenRouter specifics it handles: error chunks are propagated as a ValueError (matching _stream); top-level usage is accumulated; and OpenRouter's cost surfacing (cost/cost_details) plus native_finish_reason/model_name/etc. are carried into message-finish metadata, matching the compat bridge.

The win over the bridge

OpenRouter's chunk parser leaves reasoning in additional_kwargs; the native converter surfaces it as a reasoning block (and tool calls as tool_call blocks).

Worth careful review

• Error-chunk propagation and the cost/finish metadata parity with _stream (a test asserts cost survives to message-finish).
• The _StreamState helper shares per-chunk coercion/usage/error logic across the sync/async twins (no event-sequence drift).
• Scope cut: reasoning_details not yet surfaced as blocks (noted in-code).
• message_id keyword-only; message-start carries the LangChain run id.

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Cloned from langchain-ai#38030 for DevAsign stress testing.

[devasign-test-app]

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