fix(cassandra): auto-recover session after Cassandra restart

[dpol1]

Purpose of the PR

closes #2740

HugeGraphServer stops responding after Cassandra is restarted and never
recovers without a full server restart.

Root cause: CassandraSessionPool builds the Datastax Cluster without a
ReconnectionPolicy, CassandraSession.execute(...) calls the driver once
with no retry, and thread-local sessions are never probed for liveness.
Once Cassandra goes down, transient NoHostAvailableException /
OperationTimedOutException errors surface to the user and the pool stays
dead even after Cassandra comes back online.

Main Changes

• Register ExponentialReconnectionPolicy(baseDelay, maxDelay) on the
  Cluster builder so the Datastax driver keeps retrying downed nodes in
  the background.

• Wrap every Session.execute(...) in executeWithRetry(Statement) with
  exponential backoff on transient connectivity failures.

• Implement reconnectIfNeeded() / reset() on CassandraSession so the
  pool reopens closed sessions and issues a lightweight health-check
  (SELECT now() FROM system.local) before subsequent queries.

• Add four tunables in CassandraOptions (defaults preserve previous
  behavior for healthy clusters):

  Option	Default	Meaning
  cassandra.reconnect_base_delay	1000 ms	Initial backoff for driver reconnection policy
  cassandra.reconnect_max_delay	60000 ms	Cap for reconnection backoff
  cassandra.reconnect_max_retries	10	Per-query retries on transient errors (0 disables)
  cassandra.reconnect_interval	5000 ms	Base interval for per-query exponential backoff

• Add unit tests covering defaults, overrides, disabling retries and option keys.

Verifying these changes

• Need tests and can be verified as follows:
  • mvn -pl hugegraph-server/hugegraph-test -am test -Dtest=CassandraTest — 13/13 pass

Does this PR potentially affect the following parts?

• Modify configurations

Documentation Status

• Doc - TODO

[imbajin]

⚠️ commitAsync() bypasses retry — still calls this.session.executeAsync(s) directly

The PR wraps execute() and commit() with executeWithRetry, but commitAsync() (line 177 in the base file) still calls this.session.executeAsync(s) directly. If a Cassandra restart happens during an async batch commit, the same connectivity failure will surface without any retry.

Consider wrapping the async path as well, or at minimum adding a TODO/comment explaining why async commits are deliberately left un-retried (e.g., if retry semantics for async batches are too complex for this PR).

[dpol1]

Thanks @imbajin for the feedback, changed!