fix: implement WriteApi.flush() for batching mode

[sankalpsthakur]

Proposed Changes

WriteApi.flush() was documented as a stable API but implemented as a TODO / pass no-op (#697).

This implements flush() for batching mode by adding a flush boundary subject that forces the current batch window to close and send pending points over HTTP — the same approach used by the C# and Java client batch writers.

Behavior

• Forces immediate write of any buffered data (does not wait for batch_size or flush_interval)
• Does not dispose the write pipeline — WriteApi remains usable after flush() (unlike close())
• No-op for synchronous / asynchronous write types

Implementation notes

• Pipeline uses ops.publish + ops.window(rx.merge(time/count windows, flush_subject)), mirroring C# WriteApi / Java FlowableBufferTimedFlushable
• Empty flushes do not send HTTP requests (existing batch.size > 0 filter)

Fixes #697

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests/test_WriteApiBatching.py completes successfully (new flush tests + existing batching tests)
• Commit messages are conventional
• Sign CLA (if not already signed)

Tests

pytest tests/test_WriteApiBatching.py -v --timeout=60

New coverage:

• test_flush_partial_batch — single point under batch_size is written on flush()
• test_flush_allows_reuse — further writes work after flush()
• test_flush_empty_is_noop — empty buffer does not HTTP
• test_flush_synchronous_is_noop — sync mode does not raise

AI disclosure

AI assistance was used to locate the incomplete flush() stub, compare C#/Java batch-writer flush designs, draft the Rx boundary-window change, and write unit tests. The change was reviewed, kept scoped to the batching pipeline + tests + changelog, and verified with the batching unit tests above.

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.43%. Comparing base (9ca599f) to head (1a8f07a).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage   90.38%   90.43%   +0.04%     
==========================================
  Files          40       40              
  Lines        3546     3554       +8     
==========================================
+ Hits         3205     3214       +9     
+ Misses        341      340       -1     

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