fix(pool): sync anthropic entry on access_token change, not just refresh_token

`_sync_anthropic_entry_from_credentials_file` only checked whether the
refresh_token in ~/.claude/.credentials.json differed from the pool
entry's refresh_token.  This missed the case where the CLI performs a
silent access-token re-issue — returning a new access_token alongside
the *same* refresh_token.  The pool entry's stale bearer token was never
updated, causing 401 errors on every request until the exhausted-TTL
(5 min) expired.

Bring this function to parity with its Codex and xAI OAuth siblings:
- Check either access_token *or* refresh_token changed (dual-field guard).
- Use `file_X or entry.X` fallbacks so a partial file can't blank a field.
- Clear all six status/error fields on sync (last_error_reason,
  last_error_message, last_error_reset_at were previously omitted),
  ensuring an exhausted entry becomes available immediately.

Spotted via parity review against commit 569bc94b5 which fixed the same
pattern in `_sync_nous_entry_from_auth_store`.

agent/credential_pool.py

@@ -616,17 +616,32 @@ class CredentialPool:
             file_refresh = creds.get("refreshToken", "")
             file_access = creds.get("accessToken", "")
             file_expires = creds.get("expiresAt", 0)
-            # If the credentials file has a different token pair, sync it
-            if file_refresh and file_refresh != entry.refresh_token:
-                logger.debug("Pool entry %s: syncing tokens from credentials file (refresh token changed)", entry.id)
+            # Sync when either token changed.  Access tokens can be re-issued
+            # without a new refresh token (silent re-issue path), so checking
+            # only refresh_token misses that case and leaves a stale
+            # access_token in the pool → 401 on every request until the pool
+            # entry's exhausted TTL expires.
+            entry_access = entry.access_token or ""
+            entry_refresh = entry.refresh_token or ""
+            if (file_access or file_refresh) and (
+                (file_access and file_access != entry_access)
+                or (file_refresh and file_refresh != entry_refresh)
+            ):
+                logger.debug(
+                    "Pool entry %s: syncing tokens from credentials file (tokens changed)",
+                    entry.id,
+                )
                 updated = replace(
                     entry,
-                    access_token=file_access,
-                    refresh_token=file_refresh,
-                    expires_at_ms=file_expires,
+                    access_token=file_access or entry.access_token,
+                    refresh_token=file_refresh or entry.refresh_token,
+                    expires_at_ms=file_expires or entry.expires_at_ms,
                     last_status=None,
                     last_status_at=None,
                     last_error_code=None,
+                    last_error_reason=None,
+                    last_error_message=None,
+                    last_error_reset_at=None,
                 )
                 self._replace_entry(entry, updated)
                 self._persist()