feat: create-messages-native.ts — Anthropic pass-through service client

[HXYerror]

Part of #38.

Background

api.githubcopilot.com/v1/messages natively accepts Anthropic Messages API requests for Claude 4.5+ models. We need a new service module that forwards requests directly, bypassing the OpenAI translation layer.

Goal

src/services/copilot/create-messages-native.ts — sends an Anthropic Messages payload straight to ${copilotBaseUrl(state)}/v1/messages and returns the raw Anthropic response.

Tasks

• Create src/services/copilot/create-messages-native.ts:
  • Accept AnthropicMessagesPayload (from anthropic-types.ts) as input — no transformation
  • Build Copilot auth headers the same way other service modules do (Authorization Bearer, copilot-integration-id, editor-version, user-agent, x-github-api-version, x-request-id, x-vscode-user-agent-library-version)
  • Forward client's anthropic-version header if present (default: "2023-06-01")
  • POST to ${copilotBaseUrl(state)}/v1/messages
  • Non-streaming: return raw parsed AnthropicMessagesResponse JSON
  • Streaming: return the raw SSE byte stream to be piped directly to the client (no re-parsing needed — Copilot SSE events are already in Anthropic format)
• Add NATIVE_ANTHROPIC_MODELS constant (initial hardcoded set; replaced by dynamic detection in feat: Dynamic NATIVE_ANTHROPIC_MODELS detection from /models endpoint (vendor-based) #42):

  export const NATIVE_ANTHROPIC_MODELS = new Set([
    "claude-sonnet-4.5", "claude-sonnet-4.6",
    "claude-opus-4.5",   "claude-opus-4.6", "claude-opus-4.6-1m",
    "claude-haiku-4.5",
  ])

• Error handling: surface upstream errors using existing HTTPError class (src/lib/error.ts)
• Honor state.showToken for debug logging (same pattern as other service modules)

Upstream SSE events returned (verified by live test)

event: message_start
event: content_block_start
event: content_block_delta   (type: text_delta | thinking_delta | signature_delta)
event: content_block_stop
event: message_delta         (includes copilot_usage + amazon-bedrock-invocationMetrics)
event: message_stop
data: [DONE]

Note: upstream returns data: [DONE] at the end (same as OpenAI). Pass it through — Anthropic SDK clients ignore it.

Acceptance criteria

• createMessagesNative({model: "claude-sonnet-4.5", ...}) returns a valid AnthropicMessagesResponse with content array
• When stream: true, the client receives SSE events with correct event: prefix lines
• Thinking blocks with signature are preserved byte-exact in pass-through
• No upstream call made for non-native models (checked by caller via NATIVE_ANTHROPIC_MODELS)

File pointers

• New: src/services/copilot/create-messages-native.ts
• Reference: src/services/copilot/create-chat-completions.ts (header pattern)
• Touch: src/routes/messages/anthropic-types.ts (may need signature field added to AnthropicThinkingBlock)

[HXYerror]

Implemented in src/services/copilot/create-messages-native.ts:

• createMessagesNative(payload): forwards Anthropic payload directly to ${copilotBaseUrl}/v1/messages
• buildNativeHeaders(): adds anthropic-version: 2023-06-01 + anthropic-beta: interleaved-thinking-2025-05-14,prompt-caching-2024-07-31
• buildUpstreamPayload(): handles thinking format negotiation (legacy vs adaptive)
• isAdaptiveThinkingModel(): regex-based detection of opus-4.7+ needing adaptive format
• messageHasImages(): sets vision header when needed
• Returns async SSE iterable (streaming) or JSON object (non-streaming)

Closes #39