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.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.42%. Comparing base (bc6886d) to head (b4d85ab).

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     72.42%   72.42%           
- Complexity     3522     3529    +7     
=========================================
  Files           391      392    +1     
  Lines         16341    16368   +27     
  Branches       1702     1707    +5     
=========================================
+ Hits          11835    11855   +20     
- Misses         3867     3871    +4     
- Partials        639      642    +3     

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