Improving topic write buffer capacity accounting

[Myllyenko]

This PR minimizes probability of memory overconsumption in topic write sessions.

1. Reservation before compression

• Old behavior: The writer reserved space based on the uncompressed input size. Compression framing and potential expansion were not considered during admission.
• New behavior: The writer reserves space using the codec-provided encoded-size bound, capped at the configured buffer size.

2. Accounting for encoded-output expansion

• Old behavior: When encoded output was larger than its original reservation, no additional permits were acquired. Buffer accounting continued to use the original reservation.

• New behavior: After encoding, BufferManager#updateMessageSize attempts to account for as much of the actual encoded memory as current capacity permits without failing the message:

  • It acquires the complete additional reservation when enough permits are available.
  • Otherwise, it acquires all currently available permits.

  When the full reservation cannot be acquired, a warning reports that the configured buffer size may be temporarily exceeded. updateMessageSize now returns the effective reservation instead of void.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.16%. Comparing base (d77889d) to head (7fe40a3).

Files with missing lines	Patch %	Lines
.../java/tech/ydb/topic/write/impl/BufferManager.java	78.57%	2 Missing and 1 partial ⚠️
...in/java/tech/ydb/topic/write/impl/WriterQueue.java	92.30%	1 Missing and 1 partial ⚠️
...in/java/tech/ydb/topic/impl/ZstdBackwardCodec.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #711      +/-   ##
============================================
- Coverage     73.18%   73.16%   -0.03%     
- Complexity     3589     3599      +10     
============================================
  Files           392      393       +1     
  Lines         16531    16558      +27     
  Branches       1736     1741       +5     
============================================
+ Hits          12099    12115      +16     
- Misses         3811     3819       +8     
- Partials        621      624       +3     

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