fix(java): let the checkpoint loop own its retry budget and decode LSM stats strictly

Two fixes to the Java MemWAL LSM surface, both reproduced against the
scripted test server before being fixed.

Transport retries. `HttpClients.createDefault()` installs Apache's default
response retry strategy, whose retryable-status list is exactly 429 and 503
— the two statuses `LanceDbTableLsm.isRetryable` owns. Every explicit retry
budget in `checkpointLsm` was therefore doubled on the wire (a 429 held
against flush issued 18 requests, not 9), and `compactLsm` was retried in
place despite the loop being built to fall through to a fresh stats poll.
Automatic retries are now disabled, so the checkpoint loop is the sole owner
of the 421/429/503 transitions.

Stats decoding. `getLsmStats` read the response with Jackson's `path()`,
which yields a missing node that iterates as an empty array. That made
"malformed" indistinguishable from "no buckets", which is indistinguishable
from "drained" — so an empty response body, a `{"lsm_stats": {}}`, or a
bucket missing its required fields all made `checkpointLsm()` report
convergence for a checkpoint that never ran.

Stats now decode into `LsmStats`, `BucketStats`, `GenerationStats` and
`MemtableStats`, mirroring the Rust structs in
`rust/lancedb/src/table/lsm_stats.rs` and the objects the Node binding
already exposes. Decoding is strict and fails closed, matching the serde
contract on the Rust side: absent or null `lsm_stats` means the LSM write
path is off, and anything else present must decode into the full
bucket/generation shape. `newestGeneration` and `outstandingGenerations`
move onto `BucketStats`, matching Rust's `impl BucketStats`.

This changes `getLsmStats` from `Optional<JsonNode>` to `Optional<LsmStats>`,
which also brings Java to the typed surface Node already had rather than
handing back a raw JSON blob.

Testing: 33 passing in lancedb-core, up from 29. The new tests pin the wire
request count against the retry budget and reject five malformed stats
payloads. `testCheckpointRetriesRetryableStatusInPlace` previously passed on
a transport-absorbed 429 and now exercises the real retry path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

java/README.md

@@ -56,7 +56,8 @@ lsm.setLsmWriteSpec(LsmWriteSpec.bucket("id", 16));
 lsm.checkpointLsm();
 
 // Inspect live per-bucket state.
-lsm.getLsmStats().ifPresent(stats -> System.out.println(stats.get("buckets")));
+lsm.getLsmStats().ifPresent(stats -> stats.buckets().forEach(bucket ->
+    System.out.println(bucket.shardId() + ": " + bucket.generations().size() + " L0 generations")));
 
 client.close();
 ```