test: 添加 Context 构建单元测试 (测试计划 03)

覆盖 stripHtmlComments、isMemoryFilePath、getLargeMemoryFiles、
buildEffectiveSystemPrompt 等函数，共 25 个测试用例全部通过。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/__tests__/systemPrompt.test.ts

@@ -0,0 +1,88 @@
+import { describe, expect, test } from "bun:test";
+import { buildEffectiveSystemPrompt } from "../systemPrompt";
+
+const defaultPrompt = ["You are a helpful assistant.", "Follow instructions."];
+
+function buildPrompt(overrides: Record<string, unknown> = {}) {
+  return buildEffectiveSystemPrompt({
+    mainThreadAgentDefinition: undefined,
+    toolUseContext: { options: {} as any },
+    customSystemPrompt: undefined,
+    defaultSystemPrompt: defaultPrompt,
+    appendSystemPrompt: undefined,
+    ...overrides,
+  });
+}
+
+describe("buildEffectiveSystemPrompt", () => {
+  test("returns default system prompt when no overrides", () => {
+    const result = buildPrompt();
+    expect(Array.from(result)).toEqual(defaultPrompt);
+  });
+
+  test("overrideSystemPrompt replaces everything", () => {
+    const result = buildPrompt({ overrideSystemPrompt: "override" });
+    expect(Array.from(result)).toEqual(["override"]);
+  });
+
+  test("customSystemPrompt replaces default", () => {
+    const result = buildPrompt({ customSystemPrompt: "custom" });
+    expect(Array.from(result)).toEqual(["custom"]);
+  });
+
+  test("appendSystemPrompt is appended after main prompt", () => {
+    const result = buildPrompt({ appendSystemPrompt: "appended" });
+    expect(Array.from(result)).toEqual([...defaultPrompt, "appended"]);
+  });
+
+  test("agent definition replaces default prompt", () => {
+    const agentDef = {
+      getSystemPrompt: () => "agent prompt",
+      agentType: "custom",
+    } as any;
+    const result = buildPrompt({ mainThreadAgentDefinition: agentDef });
+    expect(Array.from(result)).toEqual(["agent prompt"]);
+  });
+
+  test("agent definition with append combines both", () => {
+    const agentDef = {
+      getSystemPrompt: () => "agent prompt",
+      agentType: "custom",
+    } as any;
+    const result = buildPrompt({
+      mainThreadAgentDefinition: agentDef,
+      appendSystemPrompt: "extra",
+    });
+    expect(Array.from(result)).toEqual(["agent prompt", "extra"]);
+  });
+
+  test("override takes precedence over agent and custom", () => {
+    const agentDef = {
+      getSystemPrompt: () => "agent prompt",
+      agentType: "custom",
+    } as any;
+    const result = buildPrompt({
+      mainThreadAgentDefinition: agentDef,
+      customSystemPrompt: "custom",
+      appendSystemPrompt: "extra",
+      overrideSystemPrompt: "override",
+    });
+    expect(Array.from(result)).toEqual(["override"]);
+  });
+
+  test("returns array of strings", () => {
+    const result = buildPrompt();
+    expect(Array.isArray(result)).toBe(true);
+    for (const item of result) {
+      expect(typeof item).toBe("string");
+    }
+  });
+
+  test("custom + append combines both", () => {
+    const result = buildPrompt({
+      customSystemPrompt: "custom",
+      appendSystemPrompt: "extra",
+    });
+    expect(Array.from(result)).toEqual(["custom", "extra"]);
+  });
+});