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

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

web/src/lib/openai-prompt-cache.ts

@@ -0,0 +1,17 @@
+export const OPENAI_PROMPT_CACHE_KEY_MAX_LENGTH = 64;
+
+export function clampOpenAIPromptCacheKey(value?: string) {
+    const key = value?.trim();
+    if (!key) return undefined;
+    return Array.from(key).slice(0, OPENAI_PROMPT_CACHE_KEY_MAX_LENGTH).join("");
+}
+
+export function canvasAgentPromptCacheKey(sessionId: string) {
+    const value = sessionId.trim();
+    return value ? clampOpenAIPromptCacheKey(`canvas-agent:${value}`) : undefined;
+}
+
+export function withOpenAIPromptCacheKey<T extends Record<string, unknown>>(body: T, value?: string): T & { prompt_cache_key?: string } {
+    const key = clampOpenAIPromptCacheKey(value);
+    return key ? { ...body, prompt_cache_key: key } : body;
+}