fix(opencode): route Muse Spark 1.2 models to OpenAI Responses API (#10874)

OpenCode Zen serves muse-spark-1.2 and muse-spark-1.2-contributor-free only on the OpenAI Responses API endpoint, not /chat/completions. Declares targetFormat: "openai-responses" for both so requests route correctly instead of returning null/empty content. Closes #10867.

Validated in an isolated worktree boarded onto origin/release/v3.8.50 (0 conflicts, 1 file):
- Added a TDD regression test (tests/unit/opencode-muse-spark-responses-10867.test.ts) since the PR had none — confirmed RED against origin/release/v3.8.50 (entries absent) and GREEN on this branch, pushed fix-in-place.
- check-file-size, check-changelog-integrity: OK.
- typecheck:core: clean.
- check-complexity / check-cognitive-complexity: OK, both under baseline.

Co-authored-by: zoser69 <zoser69@users.noreply.github.com>

open-sse/config/providers/registry/opencode/index.ts

@@ -22,6 +22,26 @@ export const opencodeProvider: RegistryEntry = {
       supportsReasoning: true,
       interleavedField: "reasoning_content",
     },
+    // #MUSE_SPARK: Muse Spark is served by OpenCode Zen ONLY on the OpenAI
+    // Responses API (https://opencode.ai/zen/v1/responses), not /chat/completions
+    // (confirmed in the official OpenCode Zen docs: https://opencode.ai/docs/zen/).
+    // Without targetFormat:"openai-responses" these models fall through to the
+    // default chat/completions pass-through and the upstream returns null/empty
+    // content (see issue #10867). The opencode provider is passthrough, so
+    // declaring them here only sets the wire format / capability flags — the
+    // live upstream model list already advertises both ids.
+    {
+      id: "muse-spark-1.2",
+      name: "Muse Spark 1.2",
+      supportsReasoning: true,
+      targetFormat: "openai-responses",
+    },
+    {
+      id: "muse-spark-1.2-contributor-free",
+      name: "Muse Spark 1.2 Contributor Free",
+      supportsReasoning: true,
+      targetFormat: "openai-responses",
+    },
     { id: "deepseek-v4-flash-free", name: "DeepSeek V4 Flash Free", supportsReasoning: true },
     // #6998: 2026-07-14 refresh — the upstream free tier rotated its lineup;
     // minimax-m3-free, minimax-m2.5-free, ling-2.6-1t-free,

tests/unit/opencode-muse-spark-responses-10867.test.ts

@@ -0,0 +1,20 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { opencodeProvider } from "../../open-sse/config/providers/registry/opencode/index.ts";
+
+// #10867: Muse Spark is served by OpenCode Zen only on the OpenAI Responses API
+// (/responses), not /chat/completions. Without targetFormat:"openai-responses"
+// these models fall through to the default chat/completions pass-through and the
+// upstream returns null/empty content.
+test("muse-spark-1.2 and muse-spark-1.2-contributor-free route to the Responses API", () => {
+  for (const id of ["muse-spark-1.2", "muse-spark-1.2-contributor-free"]) {
+    const model = opencodeProvider.models.find((m) => m.id === id);
+    assert.ok(model, `${id} should be registered in the opencode provider`);
+    assert.equal(
+      model?.targetFormat,
+      "openai-responses",
+      `${id} must target the Responses API, not the default chat/completions pass-through`
+    );
+    assert.equal(model?.supportsReasoning, true);
+  }
+});