Fix concurrency, exception-safety, and correctness bugs in JNI layer …

[vaiju1981]

…and Java API

JNI / native fixes:

• Guard rerank / embedding / streaming to_json() calls against uncaught C++ exceptions crossing the JNI boundary, which would otherwise abort the JVM.
• Hold a jllama_context user reference for the attached NativeServer worker so closing the model cannot free the context out from under the running HTTP frontend (use-after-free).
• Fix close()-during-inference hang: add a closing flag that unblocks readers parked in next()/wait_for_all(), and decrement the user count under the context mutex so the teardown wait cannot free the mutex/condvar before the releaser notifies (latent use-after-free in the ref-count itself).
• Fix NativeServer jstring_to_utf8 calling getBytes with a Charset argument while resolving the String overload (deterministic break on every server start); cache the JNI IDs thread-safely.
• Initialize parse_string_array slots to nullptr so an OOM path cannot free uninitialized memory.
• Add a pending-exception check in parse_jstring.

Java fixes:

• Widen token counts to long in the protocol translators and ChatResponseParser.extractUsageField to avoid truncation above ~2.1B tokens.
• Honor a pre-cancelled CancellationToken in complete() (stop resetting it at entry); fix completeAsJson javadoc (the params object is not mutated).

Tests and docs:

• Extract the OuteTTS codec filter into a testable helper; add rerank bounds and codec-filter unit tests, and a model-gated close()-during-inference test.
• Remove the dead BUILD_SHARED_LIBS ON line in CMakeLists.txt.
• Update plan and TODO docs to reflect the real teardown design.

Summary

Test plan

• [ X] Affected unit / integration tests pass locally
• CI is green on this branch
• [ X] Docs / CHANGELOG updated where applicable

Related issues / PRs

Checklist

• [ X] I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md
• [ X] My commits follow Conventional Commits
• [ X] No security-sensitive changes (if there are, I have notified the maintainer privately per SECURITY.md)