Add Flush(context.Context) to BulkIndexer; replace channel with ring …

[LeeFred3042U]

Description

This PR implements a Flush(context.Context) error method on the BulkIndexer interface to allow callers to drain all submitted items without closing the indexer.

These were replaced:

• The internal channel queue (bi.queue) was replaced with a lock-free ring buffer backed by three monotonic watermark counters (claimSeq, publishSeq, commitSeq).
• Per-worker channel loops (worker.run) were removed in favor of a single, ordered sequencer goroutine.

Here's what changed:

• Close() now sets an atomic closing flag and delegates directly to Flush(ctx) rather than closing a channel. This eliminates the "send on closed channel" panic if Add() is called after Close().
• Add() now safely checks the closing flag and returns an error on a closed indexer, rather than panicking.
• Context cancellation paths in Close() and Add() have been preserved and made fully deterministic.

here was what was added:

• QueueSize field to BulkIndexerConfig (to size the underlying ring buffer, internally rounding up to the next power of two).
• 6 new deterministic lifecycle tests in bulk_indexer_internal_test.go to verify concurrent flushing, reuse, and context cancellations.
• A Flush release note entry to CHANGELOG.md.

Issues Resolved

Closes [#336]

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[codecov]

Codecov Report

❌ Patch coverage is 90.35088% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.75%. Comparing base (ca3b71a) to head (1458208).
⚠️ Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
opensearchutil/bulk_indexer.go	90.35%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #930      +/-   ##
==========================================
+ Coverage   48.70%   48.75%   +0.05%     
==========================================
  Files         637      637              
  Lines       61404    61459      +55     
==========================================
+ Hits        29904    29963      +59     
+ Misses      29749    29746       -3     
+ Partials     1751     1750       -1     

Flag	Coverage Δ
integration	28.91% <61.40%> (+0.06%)	⬆️
unit	47.37% <90.35%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
opensearchutil/bulk_indexer.go	89.94% <90.35%> (+1.95%)	⬆️

... and 6 files with indirect coverage changes

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