refactor(open-sse): dedup format default in PROVIDERS via resolver (C1)

Wrap PROVIDERS in defineProviders() that appends format:"openai" when omitted,
removing 72 repeated `format:"openai"` lines. Output stays byte-for-byte
identical (verified by tests/__baseline__/verify-providers.mjs deep-equal
against snapshot). No runtime fields added; consumers unchanged.

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

tests/__baseline__/snapshot-providers.mjs

@@ -0,0 +1,11 @@
+// Snapshot current PROVIDERS output to JSON (run on OLD code before refactor).
+// Usage: node tests/__baseline__/snapshot-providers.mjs
+import { writeFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+import { PROVIDERS } from "../../open-sse/config/providers.js";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const out = join(here, "providers-baseline.json");
+writeFileSync(out, JSON.stringify(PROVIDERS, null, 2));
+console.log(`Snapshot ${Object.keys(PROVIDERS).length} providers → ${out}`);