fix(sdk): use dynamic port instead of hardcoded 4096 (fixes #2311)

- TmuxSessionManager resolves server URL with priority:
  ctx.serverUrl.origin > getServerBaseUrl(client) > OPENCODE_PORT env > localhost:4096
- Inject resolved baseUrl into SDK client (root + session subclient) at
  startup so all session tools use the dynamic port
- Extract shared getInternalClient utility to sdk-internal-client.ts;
  prefer candidate with getConfig/setConfig for correct v1/v2 detection
- Support v1 (client._client) and v2 (client.client) SDK structures
  across opencode-server-auth.ts and opencode-http-api.ts
- Use createMockContext factory for serverUrl overrides in tests

src/index.ts

@@ -12,7 +12,7 @@ import { createPluginDispose, type PluginDispose } from "./plugin-dispose"
 import { loadPluginConfig } from "./plugin-config"
 import { createModelCacheState } from "./plugin-state"
 import { createFirstMessageVariantGate } from "./shared/first-message-variant"
-import { injectServerAuthIntoClient, log } from "./shared"
+import { injectServerAuthIntoClient, injectServerBaseUrlIntoClient, log } from "./shared"
 import { startTmuxCheck } from "./tools"
 
 let activePluginDispose: PluginDispose | null = null
@@ -24,6 +24,12 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     directory: ctx.directory,
   })
 
+  // Inject correct server baseUrl to ensure SDK client uses dynamic port
+  // Priority: ctx.serverUrl > fallback to existing client baseUrl
+  if (ctx.serverUrl) {
+    injectServerBaseUrlIntoClient(ctx.client, ctx.serverUrl.toString())
+  }
+
   injectServerAuthIntoClient(ctx.client)
   startTmuxCheck()
   await activePluginDispose?.()