refactor(open-sse): add provider/model schema skeleton (C-prep)

New unwired modules providers/schema.js + models/schema.js with PROVIDER_DEFAULTS,
ENDPOINT_DEFAULTS, resolveProvider, MODEL_DEFAULTS, resolveModel (3-tier merge).
Foundation for upcoming registry tasks; no runtime wiring yet.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/providers/schema.js

@@ -0,0 +1,31 @@
+// Provider transport schema: shared defaults + endpoint defaults + resolver (skeleton, not wired)
+import { DEFAULT_RETRY_CONFIG, FETCH_CONNECT_TIMEOUT_MS } from "../config/runtimeConfig.js";
+
+// Shared defaults: provider only overrides fields that differ
+export const PROVIDER_DEFAULTS = {
+  format: "openai",
+  headers: {},
+  auth: { header: "Authorization", scheme: "bearer", source: ["accessToken", "apiKey"] },
+  retry: DEFAULT_RETRY_CONFIG,
+  timeoutMs: FETCH_CONNECT_TIMEOUT_MS,
+  executor: "default"
+};
+
+// Default endpoints per format (provider only overrides what differs)
+export const ENDPOINT_DEFAULTS = {
+  openai: { chat: "/chat/completions", test: "/models", models: "/models" },
+  claude: { chat: "/messages", test: "/models", countTokens: "/messages/count_tokens" },
+  gemini: { chat: "/{model}:streamGenerateContent", models: "/models", test: "/models" }
+};
+
+// Deep-merge a provider entry over PROVIDER_DEFAULTS (defensive for missing transport)
+export function resolveProvider(entry) {
+  const transport = (entry && entry.transport) || {};
+  return {
+    ...PROVIDER_DEFAULTS,
+    ...transport,
+    headers: { ...PROVIDER_DEFAULTS.headers, ...transport.headers },
+    auth: { ...PROVIDER_DEFAULTS.auth, ...transport.auth },
+    retry: { ...PROVIDER_DEFAULTS.retry, ...transport.retry }
+  };
+}