SwiftInferChatModel class

A dartantic ChatModel that speaks swift-infer's Anthropic-compatible /v1/messages wire (ADR 0003, lenny-4dhv.1).

Why a custom model instead of dartantic's stock AnthropicChatModel:

• swift-infer/Qwen streams thinking content blocks without Anthropic's cryptographic signature. anthropic_sdk_dart's strict parser hard-casts signature as a non-null String and throws every turn. This model parses the SSE leniently and never requires a signature.
• swift-infer accepts Qwen-tuned sampling knobs (top_k, presence_penalty, repetition_penalty, reasoning_effort, preserve_thinking) that AnthropicChatOptions cannot express. Unset knobs are omitted so the node's model-card defaults apply.

Streaming convention mirrors dartantic's own Anthropic mapper: each emitted ChatResult's output is a delta ChatMessage carrying the incremental part(s) — a ThinkingPart for reasoning deltas, a TextPart for answer text, and a single ToolPart.call emitted at content_block_stop once its input_json has fully accumulated. finishReason rides the message_delta. A higher layer (dartantic's Agent/accumulator, or lenny's ModelProvider seam) consolidates the deltas.

swift-infer / Qwen quirk: reasoning can arrive either as native Anthropic thinking/thinking_delta blocks OR inlined as <think>...</think> markers inside text_delta. Both are routed to ThinkingPart; the <think> scan is stateful across chunks (including markers split across chunk boundaries). The marker handling is inbound only — prior reasoning is always replayed outbound as a native thinking content block.

Tool names are passed through verbatim and must match Anthropic's ^[a-zA-Z0-9_-]{1,64}$ (no dots). lenny's dotted tool namespacing (core.tap) is encoded/decoded by the ModelProvider seam, not here, so this generic model stays backend-agnostic.

Web-compatible: pure Dart over package:http, no dart:io. Does not retry internally — it uses the supplied client directly, so the caller owns retry/timing (the lenny SchemaRejection/runaway-cap policy lives in the seam, not here).