fix: detect GPT models behind proxy providers (litellm, ollama) in isGptModel

isGptModel only matched openai/ and github-copilot/gpt- prefixes, causing
models like litellm/gpt-5.2 to fall into the Claude code path. This
injected Claude-specific thinking config, which the opencode runtime
translated into a reasoningSummary API parameter — rejected by OpenAI.

Extract model name after provider prefix and match against GPT model
name patterns (gpt-*, o1, o3, o4).

Closes #1788

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/types.test.ts

@@ -0,0 +1,49 @@
+import { describe, test, expect } from "bun:test";
+import { isGptModel } from "./types";
+
+describe("isGptModel", () => {
+  test("standard openai provider models", () => {
+    expect(isGptModel("openai/gpt-5.2")).toBe(true);
+    expect(isGptModel("openai/gpt-4o")).toBe(true);
+    expect(isGptModel("openai/o1")).toBe(true);
+    expect(isGptModel("openai/o3-mini")).toBe(true);
+  });
+
+  test("github copilot gpt models", () => {
+    expect(isGptModel("github-copilot/gpt-5.2")).toBe(true);
+    expect(isGptModel("github-copilot/gpt-4o")).toBe(true);
+  });
+
+  test("litellm proxied gpt models", () => {
+    expect(isGptModel("litellm/gpt-5.2")).toBe(true);
+    expect(isGptModel("litellm/gpt-4o")).toBe(true);
+    expect(isGptModel("litellm/o1")).toBe(true);
+    expect(isGptModel("litellm/o3-mini")).toBe(true);
+    expect(isGptModel("litellm/o4-mini")).toBe(true);
+  });
+
+  test("other proxied gpt models", () => {
+    expect(isGptModel("ollama/gpt-4o")).toBe(true);
+    expect(isGptModel("custom-provider/gpt-5.2")).toBe(true);
+  });
+
+  test("gpt4 prefix without hyphen (legacy naming)", () => {
+    expect(isGptModel("litellm/gpt4o")).toBe(true);
+    expect(isGptModel("ollama/gpt4")).toBe(true);
+  });
+
+  test("claude models are not gpt", () => {
+    expect(isGptModel("anthropic/claude-opus-4-6")).toBe(false);
+    expect(isGptModel("anthropic/claude-sonnet-4-5")).toBe(false);
+    expect(isGptModel("litellm/anthropic.claude-opus-4-5")).toBe(false);
+  });
+
+  test("gemini models are not gpt", () => {
+    expect(isGptModel("google/gemini-3-pro")).toBe(false);
+    expect(isGptModel("litellm/gemini-3-pro")).toBe(false);
+  });
+
+  test("opencode provider is not gpt", () => {
+    expect(isGptModel("opencode/claude-opus-4-6")).toBe(false);
+  });
+});