fix(node-sdk): prevent SDK from forcing application shutdown

[roncohen]

Summary

• Remove SDK signal handlers and forced process termination so applications retain ownership of graceful shutdown, regardless of handler registration order.
• Keep best-effort automatic flushing on natural event-loop shutdown (beforeExit).
• Only report interrupted automatic flushes, avoiding false warnings when applications flush explicitly before exiting.
• Update API documentation, README, and changeset.

Trade-off / migration

Installing a Node signal listener suppresses default termination, so removing only process.exit() can leave applications running indefinitely. This change removes signal interception altogether.

Automatic flushing on termination signals is no longer provided. Applications should await client.flush() in their existing graceful shutdown hooks after draining in-flight work and before exiting. batchOptions.flushOnExit now applies only to natural event-loop shutdown.

Validation

• Node SDK: all 230 tests pass.
• Node SDK TypeScript build passes; lint and formatting checks pass.
• Real-process regression tests cover unhandled SIGTERM/SIGINT, asynchronous shutdown handlers registered before and after SDK initialization, multiple clients, exit-code preservation, and explicit flushing before exit.

Fixes BUC-4278.

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