docs: TESTING_INTERNALS notes the quota preflight's unreadable-usage path

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/TESTING_INTERNALS.md

@@ -1018,7 +1018,9 @@ first (`UBI_REAP=0` skips it). Names outside the `ubirun-` prefix are never reap
 quota preflight adds the size's vCPUs to the project's running VMs and refuses
 before the create request when that exceeds the known ceiling
 (`UBI_QUOTA_VCPUS`, or the maximum the last refusal printed, remembered in the
-state root; `UBI_QUOTA_WAIT=<seconds>` waits for room instead). `run`, `verify`
+state root; `UBI_QUOTA_WAIT=<seconds>` waits for room instead). When the usage read fails (`usage` exits 1 on an
+unreadable VM list or size and skips a VM that 404s), the preflight retries
+within `UBI_QUOTA_WAIT`, then logs and leaves the decision to the API. `run`, `verify`
 and `ssh NAME COMMAND` stream the remote output through a watchdog: a heartbeat
 line every `UBI_HEARTBEAT` seconds (300), and a command silent for
 `UBI_IDLE_TIMEOUT` seconds (1800) or older than `UBI_RUN_TIMEOUT` (0, unlimited)