feat: Route dispatch — detect native-capable models, branch to pass-through

[HXYerror]

Part of #38. Depends on #39.

Background

src/routes/messages/route.ts currently routes all /v1/messages requests through the OpenAI translation layer. We need to branch: Claude 4.5+ models → native pass-through, everything else → existing translation.

Goal

Add model-based dispatch at the top of the messages handler so native-capable Claude models skip translation entirely.

Tasks

• In src/routes/messages/handler.ts (or route.ts), after parsing the request body:

  import { NATIVE_ANTHROPIC_MODELS, createMessagesNative } from "~/services/copilot/create-messages-native"
  
  const model = resolveAlias(payload.model)  // after F1.B lands; for now use payload.model
  if (NATIVE_ANTHROPIC_MODELS.has(model)) {
    // native pass-through path
    return createMessagesNative(payload, c)
  }
  // existing translation path (non-stream-translation.ts → /chat/completions)

• Model resolution: apply alias resolution before the native check (so client can use alias "sonnet-4.5" → resolves to "claude-sonnet-4.5" → hits native path)
• If NATIVE_ANTHROPIC_MODELS.has(model) is false but model is Claude-family: fall through to translation (handles legacy claude-sonnet-4 on the translation path)
• Log the dispatch decision at --verbose level: "[messages] native pass-through for claude-sonnet-4.5" vs "[messages] translation path for claude-sonnet-4"
• Keep /v1/messages/count_tokens on the existing heuristic path (no native count_tokens endpoint on Copilot)

Acceptance criteria

• POST /v1/messages {model: "claude-sonnet-4.5", thinking: {...}} uses native path — confirmed by thinking blocks in response
• POST /v1/messages {model: "claude-sonnet-4"} uses translation path — confirmed by no thinking blocks (not supported on translation path) and OpenAI-style upstream call visible in --verbose
• No regression for non-Claude models (gpt-4o etc.) on the translation path

File pointers

• Touch: src/routes/messages/handler.ts or route.ts
• Imports from: src/services/copilot/create-messages-native.ts (feat: create-messages-native.ts — Anthropic pass-through service client #39)
• Imports from: src/lib/alias.ts (F1.B — add guard if not yet landed)

[HXYerror]

Implemented in src/routes/messages/handler.ts:

• handleCompletion() now calls isNativeAnthropicModel(payload.model) to branch between handleNative() and handleTranslated()
• handleNative(): proxies SSE events verbatim for streaming; returns c.json(response) for non-streaming
• handleTranslated(): retains the existing OpenAI translation path unchanged

Closes #40