fix(auth): improve multi-account round-robin rotation and 429 handling

This commit fixes several issues with multi-account OAuth rotation that
were causing slow responses and inefficient account cycling.

## Changes

### 1. Fix usageStats race condition (auth-profiles.ts)

The `markAuthProfileUsed`, `markAuthProfileCooldown`, `markAuthProfileGood`,
and `clearAuthProfileCooldown` functions were using a stale in-memory store
passed as a parameter. Long-running sessions would overwrite usageStats
updates from concurrent sessions when saving.

**Fix:** Re-read the store from disk before each update to get fresh
usageStats from other sessions, then merge the update.

### 2. Capture AbortError from waitForCompactionRetry (pi-embedded-runner.ts)

When a request timed out, `session.abort()` was called which throws an
`AbortError`. The code structure was:

```javascript
try {
  await session.prompt(params.prompt);
} catch (err) {
  promptError = err;  // Catches AbortError here
}
await waitForCompactionRetry();  // But THIS also throws AbortError!
```

The second `AbortError` from `waitForCompactionRetry()` escaped and
bypassed the rotation/fallback logic entirely.

**Fix:** Wrap `waitForCompactionRetry()` in its own try/catch to capture
the error as `promptError`, enabling proper timeout handling.

Root cause analysis and fix proposed by @erikpr1994 in #313.

Fixes #313

### 3. Fail fast on 429 rate limits (pi-ai patch)

The pi-ai library was retrying 429 errors up to 3 times with exponential
backoff before throwing. This meant a rate-limited account would waste
30+ seconds retrying before our rotation code could try the next account.

**Fix:** Patch google-gemini-cli.js to:
- Throw immediately on first 429 (no retries)
- Not catch and retry 429 errors in the network error handler

This allows the caller to rotate to the next account instantly on rate limit.

Note: We submitted this fix upstream (https://github.com/badlogic/pi-mono/pull/504)
but it was closed without merging. Keeping as a local patch for now.

## Testing

With 6 Antigravity accounts configured:
- Accounts rotate properly based on lastUsed (round-robin)
- 429s trigger immediate rotation to next account
- usageStats persist correctly across concurrent sessions
- Cooldown tracking works as expected

## Before/After

**Before:** Multiple 429 retries on same account, 30-90s delays
**After:** Instant rotation on 429, responses in seconds

patches/@mariozechner__pi-ai.patch

@@ -213,3 +213,31 @@ index 20fb0a22aaa28f7ff7c2f44a8b628fa1d9d7d936..31bae0aface1319487ce62d35f1f3b6e
      }));
  }
  function mapStopReason(status) {
+diff --git a/dist/providers/google-gemini-cli.js b/dist/providers/google-gemini-cli.js
+--- a/dist/providers/google-gemini-cli.js
++++ b/dist/providers/google-gemini-cli.js
+@@ -168,7 +168,12 @@ async function* streamCompletion(params, options) {
+                         break; // Success, exit retry loop
+                     }
+                     const errorText = await response.text();
+-                    // Check if retryable
++                    // PATCH: Fail immediately on 429 to let caller rotate accounts
++                    if (response.status === 429) {
++                        console.log(`[pi-ai] 429 rate limit - failing fast to rotate account`);
++                        throw new Error(`Cloud Code Assist API error (${response.status}): ${errorText}`);
++                    }
++                    // Check if retryable (non-429 errors)
+                     if (attempt < MAX_RETRIES && isRetryableError(response.status, errorText)) {
+                         // Use server-provided delay or exponential backoff
+                         const serverDelay = extractRetryDelay(errorText);
+@@ -183,6 +188,10 @@ async function* streamCompletion(params, options) {
+                     if (error instanceof Error && error.message === "Request was aborted") {
+                         throw error;
+                     }
++                    // PATCH: Don't retry 429 errors - let caller rotate accounts
++                    if (error instanceof Error && error.message.includes("429")) {
++                        throw error;
++                    }
+                     lastError = error instanceof Error ? error : new Error(String(error));
+                     // Network errors are retryable
+                     if (attempt < MAX_RETRIES) {