### What problem does this PR solve?
Resolve#14137 .
### Problem
Graph resolution succeeds (nodes/edges merged, pagerank updated), but
the subsequent burst of Infinity write operations in `set_graph`
exhausts the connection pool with `TOO_MANY_CONNECTIONS` errors. Root
causes:
1. **Hardcoded pool size** — `infinity_conn_pool.py` hardcoded
`ConnectionPool(max_size=4)` on initial creation and `max_size=32` on
refresh. Operators cannot tune this without patching code.
2. **No retry on transient failures** — a single `TOO_MANY_CONNECTIONS`
on edge deletes or chunk inserts kills the entire resolution+community
pipeline with no retry.
### Changes
#### `common/doc_store/infinity_conn_pool.py`
- Read `ConnectionPool` `max_size` from the `INFINITY_POOL_MAX_SIZE`
environment variable (default: `4`), applied consistently to both
initial creation and refresh paths.
- Log the actual pool size on startup for easier debugging.
#### `rag/graphrag/utils.py` — `set_graph()`
- **Edge deletes**: add exponential-backoff retry (3 attempts, 1s/2s/4s
delays) so transient `TOO_MANY_CONNECTIONS` errors are retried instead
of failing the entire job. Concurrency continues to be gated by the
existing `chat_limiter`.
- **Batch inserts**: add exponential-backoff retry (3 attempts, 1s/2s/4s
delays) for the same reason.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
---------
Signed-off-by: noob <yixiao121314@outlook.com>
