fix: introduce a max in progress block count that can trigger back pressure

[timfn-hg]

Description:
Create a new configuration that allows us to specify the maximum number of in-progress blocks within the block buffer to catch cases where upstream blocks are not able to be closed for some reason. In this case, we want to trigger back pressure sooner rather than later since it means there is a problem with the blocks and we should be more aggressive in applying back pressure to decrease the risk of data issues.

At any given moment there is always likely to be one block in progress, so applying this new configuration doesn't kick in unless at least 2 blocks are in progress. This means that aside from the latest block, there are older blocks that have yet to be closed. The threshold is currently 5 blocks.

Related issue(s):

Fixes #26949

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues

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[codecov]

Codecov Report

❌ Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../app/blocks/impl/streaming/BlockBufferService.java	89.18%	3 Missing and 1 partial ⚠️

@@             Coverage Diff              @@
##               main   #26993      +/-   ##
============================================
+ Coverage     70.55%   70.56%   +0.01%     
+ Complexity    11692    11691       -1     
============================================
  Files          2586     2586              
  Lines        108553   108570      +17     
  Branches      12165    12168       +3     
============================================
+ Hits          76586    76610      +24     
+ Misses        27970    27964       -6     
+ Partials       3997     3996       -1     

Files with missing lines	Coverage Δ	Complexity Δ
...com/hedera/node/config/data/BlockBufferConfig.java	100.00% <ø> (ø)	0.00 <0.00> (ø)
.../app/blocks/impl/streaming/BlockBufferService.java	80.40% <89.18%> (+0.70%)	0.00 <0.00> (ø)

... and 3 files with indirect coverage changes

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[ibankov]

LGTM, ty @timfn-hg. I had a small comment but not blocking

[timfn-hg]

XTS: https://github.com/hiero-ledger/hiero-consensus-node/actions/runs/33087240217 (passing)

[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 9261cae
▶️ Tests: 22785 executed
⚪️ Checks: 86/86 completed

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