fix(sse): use x-api-key for opencode-go minimax messages requests (#733)

OpenCode Go mixes request protocols by model family:

   - `glm-5` and `kimi-k2.5` use OpenAI-style `/chat/completions`
   - `minimax-m2.5` and `minimax-m2.7` use Anthropic-style `/messages`

   OmniRoute already routed MiniMax Go models to `/messages`, but the
   executor still sent `Authorization: Bearer ...`, which caused upstream
   `401 Missing API key` errors.

   This changes `OpencodeExecutor` to send:
   - `x-api-key` + `anthropic-version` for Claude-targeted OpenCode Go requests
   - `Authorization: Bearer ...` for the remaining OpenCode Go request formats

   Also updates unit coverage to assert the correct header behavior for
   MiniMax Go models.

   Validated with:
   - direct curl repro against OpenCode Go endpoints
   - `node --import tsx/esm --test tests/unit/opencode-executor.test.mjs`
   - `npm run typecheck:core`
   - `npm run build`

open-sse/executors/opencode.ts

@@ -45,7 +45,11 @@ export class OpencodeExecutor extends BaseExecutor {
     const key = credentials?.apiKey || credentials?.accessToken;
 
     if (key) {
-      headers["Authorization"] = `Bearer ${key}`;
+      if (this._requestFormat === "claude") {
+        headers["x-api-key"] = key;
+      } else {
+        headers["Authorization"] = `Bearer ${key}`;
+      }
     }
 
     if (this._requestFormat === "claude") {

tests/unit/opencode-executor.test.mjs

@@ -158,7 +158,7 @@ describe("OpencodeExecutor", () => {
       );
 
       assert.deepEqual(result.headers, {
-        Authorization: "Bearer claude-key",
+        "x-api-key": "claude-key",
         "Content-Type": "application/json",
         "anthropic-version": "2023-06-01",
         Accept: "text/event-stream",