GH-5807 Recover LMDB readers safely when maxreaders is full

[hmottestad]

GitHub issue resolved: #5807

Briefly describe the changes proposed in this PR:

Fixes #5807.

• Added a Spring Boot RDF4J Server regression test that reproduces LMDB read handle exhaustion after many timed-out queries.
• Made LMDB read transaction acquisition recover from MDB_READERS_FULL by closing pooled reset-mode readers, checking for dead LMDB readers, waiting briefly for tracked reader owners, and retrying.
• Preserved inactive/reset tracked read transactions until their owner closes them; inactive means the LMDB txn was reset, not that the Java owner is gone.
• Added focused TxnManager regressions proving reader-full recovery and manager reset do not abort a tracked inactive transaction that is still owned by a live caller.
• Added a ValueStore retry path that closes stale thread-local read transactions before retrying after reader exhaustion.

Root cause: timed-out server query work could leave LMDB read transactions occupying reader slots. Once LMDB reached maxreaders, subsequent reads failed with MDB_READERS_FULL. Reader recovery must reclaim only handles that are safe to own, such as pooled reset-mode readers or dead process/thread readers reported by LMDB; aborting a tracked inactive transaction can invalidate a live caller's handle.

Verification:

python3 .codex/skills/mvnf/scripts/mvnf.py --module core/sail/lmdb TxnManagerTest --retain-logs --stream
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS
[mvnf] Tests passed.

python3 .codex/skills/mvnf/scripts/mvnf.py --module tools/server-boot LmdbTimedOutQueryReadHandleTest#lmdbRepositoryStillAcceptsQueriesAfterManyTimedOutServerQueries --retain-logs --stream
LMDB timeout reproducer stats: timeouts=22, readerHandleFailures=0, failureSamples=[]
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS
[mvnf] Tests passed.

python3 .codex/skills/mvnf/scripts/mvnf.py core/sail/lmdb --retain-logs --stream
Tests run: 874, Failures: 0, Errors: 0, Skipped: 2
Tests run: 6, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS
[mvnf] Tests passed.

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