## Why
The per-connection limit of 128 file read handles was checked only after opening the file, leaving pending opens outside the limit.
## What changed
Reserve a semaphore permit before polling the file-open future, counting pending opens and registered handles against the same limit. Release capacity when an open fails or is cancelled, or when a handle closes, even if a read still retains the file.
Use synchronous handle-table locks that never cross an await, and recheck duplicate IDs after opening to preserve the winning handle during concurrent opens.
## Testing
Add tests for stalled-open progress, pending-open admission limits, permit cleanup after failure or cancellation, capacity reuse while a file remains in use, and concurrent duplicate IDs.
GitOrigin-RevId: caabf38c74dda5951f2fc75234a7149daaa01263
