fix(web): add prompt cache affinity to canvas agent (#74)

Co-authored-by: Tiantianr <260026737+Tiantianr@users.noreply.github.com>

web/test/agent-prompt-cache.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, test } from "bun:test";
+
+import { canvasAgentPromptCacheKey, clampOpenAIPromptCacheKey, OPENAI_PROMPT_CACHE_KEY_MAX_LENGTH, withOpenAIPromptCacheKey } from "../src/lib/openai-prompt-cache";
+
+describe("Agent prompt cache", () => {
+    test("同一会话生成稳定且隔离的缓存键", () => {
+        expect(canvasAgentPromptCacheKey("session-a")).toBe("canvas-agent:session-a");
+        expect(canvasAgentPromptCacheKey("session-a")).toBe(canvasAgentPromptCacheKey("session-a"));
+        expect(canvasAgentPromptCacheKey("session-a")).not.toBe(canvasAgentPromptCacheKey("session-b"));
+        expect(canvasAgentPromptCacheKey(" ")).toBeUndefined();
+    });
+
+    test("缓存键按 OpenAI 上限截断", () => {
+        const key = clampOpenAIPromptCacheKey(`canvas-agent:${"x".repeat(100)}`);
+        expect(Array.from(key || "")).toHaveLength(OPENAI_PROMPT_CACHE_KEY_MAX_LENGTH);
+    });
+
+    test("Responses 请求体携带缓存键", () => {
+        const body = withOpenAIPromptCacheKey({ model: "test-model", input: [] }, " canvas-agent:session-a ");
+        expect(body.prompt_cache_key).toBe("canvas-agent:session-a");
+    });
+
+    test("空缓存键不会写入 Responses 请求体", () => {
+        const body = withOpenAIPromptCacheKey({ model: "test-model", input: [] }, " ");
+        expect(body).not.toHaveProperty("prompt_cache_key");
+    });
+});