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/models/schema.js

@@ -0,0 +1,32 @@
+// Model schema (Mongoose-lite): central defaults + normalizer + 3-tier resolver (skeleton, not wired)
+
+// Centralized model defaults (caps declares positive capabilities)
+export const MODEL_DEFAULTS = {
+  type: "llm",
+  quotaFamily: "normal",
+  strip: [],
+  targetFormat: null,
+  caps: { vision: false, audioIn: false, thinking: false }
+};
+
+// Normalize a raw model entry over defaults (deep-merge caps)
+export function normalizeModel(raw) {
+  return {
+    ...MODEL_DEFAULTS,
+    ...raw,
+    caps: { ...MODEL_DEFAULTS.caps, ...raw.caps },
+    upstreamModelId: raw.upstreamModelId || raw.id
+  };
+}
+
+// 3-tier fallback: defaults -> catalog canonical -> provider inline override (wins)
+export function resolveModel(catalog, ref) {
+  const raw = typeof ref === "string" ? { id: ref } : ref;
+  return {
+    ...MODEL_DEFAULTS,
+    ...catalog[raw.id],
+    ...raw,
+    caps: { ...MODEL_DEFAULTS.caps, ...catalog[raw.id]?.caps, ...raw.caps },
+    upstreamModelId: raw.upstreamModelId || catalog[raw.id]?.upstreamModelId || raw.id
+  };
+}