feat(github): route Claude models through Copilot's native /v1/messages

GitHub Copilot's /chat/completions and /responses endpoints never surface
prompt-cache token counts for Claude models. Route Claude models (detected
by name pattern) to Copilot's Anthropic-native /v1/messages shim via a new
executeWithMessagesEndpoint(), translating OpenAI-shape requests to Claude
natively so cache_control gets injected and cached_tokens surface.

Also fixes translateRequest()'s internal _toolNameMap being sent upstream,
which made Anthropic's strict schema reject tool-call requests with a 400 —
now stripped and threaded through response state. Removes the now-dead
response_format Claude JSON-mode workaround.

open-sse/providers/registry/github.js

@@ -18,6 +18,7 @@ export default {
   transport: {
     baseUrl: "https://api.githubcopilot.com/chat/completions",
     responsesUrl: "https://api.githubcopilot.com/responses",
+    messagesUrl: "https://api.githubcopilot.com/v1/messages",
     headers: {
       "copilot-integration-id": "vscode-chat",
       "editor-version": "vscode/1.110.0",
@@ -46,6 +47,14 @@ export default {
     { id: "gpt-5.3-codex", name: "GPT-5.3 Codex" },
     { id: "gpt-5.4", name: "GPT-5.4" },
     { id: "gpt-5.4-mini", name: "GPT-5.4 Mini" },
+    // Note: routing to Copilot's Anthropic-native /v1/messages shim (see
+    // executors/github.js) is decided by model-NAME pattern at request time, not by
+    // a static targetFormat field here — Copilot's live model catalog (see
+    // services/copilotModels.js) regularly exposes claude-* models this static list
+    // hasn't caught up with yet (e.g. claude-opus-4.8), and a static per-entry
+    // targetFormat would silently miss those while also double-translating requests
+    // for models that ARE listed here (chatCore.js would pre-translate to Claude
+    // shape, then the executor would translate again). Keep these as plain entries.
     { id: "claude-haiku-4.5", name: "Claude Haiku 4.5" },
     { id: "claude-opus-4.5", name: "Claude Opus 4.5" },
     { id: "claude-sonnet-4.5", name: "Claude Sonnet 4.5" },