feat(antigravity-auth): add OAuth flow and token management

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth/antigravity/project.ts

@@ -1 +1,166 @@
-// Antigravity project context - to be implemented in Task 6
+/**
+ * Antigravity project context management.
+ * Handles fetching GCP project ID via Google's loadCodeAssist API.
+ */
+
+import {
+  ANTIGRAVITY_DEFAULT_PROJECT_ID,
+  ANTIGRAVITY_ENDPOINT_FALLBACKS,
+  ANTIGRAVITY_API_VERSION,
+  ANTIGRAVITY_HEADERS,
+} from "./constants"
+import type {
+  AntigravityProjectContext,
+  AntigravityLoadCodeAssistResponse,
+} from "./types"
+
+/**
+ * In-memory cache for project context per access token.
+ * Prevents redundant API calls for the same token.
+ */
+const projectContextCache = new Map<string, AntigravityProjectContext>()
+
+/**
+ * Client metadata for loadCodeAssist API request.
+ * Matches cliproxyapi implementation.
+ */
+const CODE_ASSIST_METADATA = {
+  ideType: "IDE_UNSPECIFIED",
+  platform: "PLATFORM_UNSPECIFIED",
+  pluginType: "GEMINI",
+} as const
+
+/**
+ * Extracts the project ID from a cloudaicompanionProject field.
+ * Handles both string and object formats.
+ *
+ * @param project - The cloudaicompanionProject value from API response
+ * @returns Extracted project ID string, or undefined if not found
+ */
+function extractProjectId(
+  project: string | { id: string } | undefined
+): string | undefined {
+  if (!project) {
+    return undefined
+  }
+
+  // Handle string format
+  if (typeof project === "string") {
+    const trimmed = project.trim()
+    return trimmed || undefined
+  }
+
+  // Handle object format { id: string }
+  if (typeof project === "object" && "id" in project) {
+    const id = project.id
+    if (typeof id === "string") {
+      const trimmed = id.trim()
+      return trimmed || undefined
+    }
+  }
+
+  return undefined
+}
+
+/**
+ * Calls the loadCodeAssist API to get project context.
+ * Tries each endpoint in the fallback list until one succeeds.
+ *
+ * @param accessToken - Valid OAuth access token
+ * @returns API response or null if all endpoints fail
+ */
+async function callLoadCodeAssistAPI(
+  accessToken: string
+): Promise<AntigravityLoadCodeAssistResponse | null> {
+  const requestBody = {
+    metadata: CODE_ASSIST_METADATA,
+  }
+
+  const headers: Record<string, string> = {
+    Authorization: `Bearer ${accessToken}`,
+    "Content-Type": "application/json",
+    "User-Agent": ANTIGRAVITY_HEADERS["User-Agent"],
+    "X-Goog-Api-Client": ANTIGRAVITY_HEADERS["X-Goog-Api-Client"],
+    "Client-Metadata": ANTIGRAVITY_HEADERS["Client-Metadata"],
+  }
+
+  // Try each endpoint in the fallback list
+  for (const baseEndpoint of ANTIGRAVITY_ENDPOINT_FALLBACKS) {
+    const url = `${baseEndpoint}/${ANTIGRAVITY_API_VERSION}:loadCodeAssist`
+
+    try {
+      const response = await fetch(url, {
+        method: "POST",
+        headers,
+        body: JSON.stringify(requestBody),
+      })
+
+      if (!response.ok) {
+        // Try next endpoint on failure
+        continue
+      }
+
+      const data =
+        (await response.json()) as AntigravityLoadCodeAssistResponse
+      return data
+    } catch {
+      // Network or parsing error, try next endpoint
+      continue
+    }
+  }
+
+  // All endpoints failed
+  return null
+}
+
+/**
+ * Fetch project context from Google's loadCodeAssist API.
+ * Extracts the cloudaicompanionProject from the response.
+ * Falls back to ANTIGRAVITY_DEFAULT_PROJECT_ID if API fails or returns empty.
+ *
+ * @param accessToken - Valid OAuth access token
+ * @returns Project context with cloudaicompanionProject ID
+ */
+export async function fetchProjectContext(
+  accessToken: string
+): Promise<AntigravityProjectContext> {
+  // Check cache first
+  const cached = projectContextCache.get(accessToken)
+  if (cached) {
+    return cached
+  }
+
+  // Call the API
+  const response = await callLoadCodeAssistAPI(accessToken)
+
+  // Extract project ID from response
+  const projectId = response
+    ? extractProjectId(response.cloudaicompanionProject)
+    : undefined
+
+  // Build result with fallback
+  const result: AntigravityProjectContext = {
+    cloudaicompanionProject: projectId || ANTIGRAVITY_DEFAULT_PROJECT_ID,
+  }
+
+  // Cache the result
+  if (projectId) {
+    projectContextCache.set(accessToken, result)
+  }
+
+  return result
+}
+
+/**
+ * Clear the project context cache.
+ * Call this when tokens are refreshed or invalidated.
+ *
+ * @param accessToken - Optional specific token to clear, or clears all if not provided
+ */
+export function clearProjectContextCache(accessToken?: string): void {
+  if (accessToken) {
+    projectContextCache.delete(accessToken)
+  } else {
+    projectContextCache.clear()
+  }
+}