fix(sdk): respect max_buffer_size when merging batches

[haubur]

Which issue does this PR address?

Closes #3932

Rationale

Respect the max_buffer_size such that the BackpressureMode can kick in.

What changed?

Merge both inner IggyMessages and the OwnedSemaphorePermits (bytes_permit), freeing only when written, not after merge.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

None

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
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See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 95.86466% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.80%. Comparing base (e80992b) to head (aba71ca).

Files with missing lines	Patch %	Lines
core/sdk/src/clients/producer_dispatcher.rs	91.02%	3 Missing and 4 partials ⚠️
core/sdk/src/clients/producer_sharding.rs	97.87%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3933      +/-   ##
============================================
- Coverage     84.95%   84.80%   -0.16%     
  Complexity     1405     1405              
============================================
  Files          1225     1225              
  Lines        179727   179932     +205     
  Branches     146014   146219     +205     
============================================
- Hits         152689   152590      -99     
- Misses        23013    23282     +269     
- Partials       4025     4060      +35     

Components	Coverage Δ
Rust Core	85.80% <95.86%> (-0.05%)	⬇️
Java SDK	67.35% <ø> (ø)
C# SDK	75.39% <ø> (+0.08%)	⬆️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	94.51% <ø> (-1.74%)	⬇️
Go SDK	69.36% <ø> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
core/sdk/src/clients/producer_sharding.rs	95.80% <97.87%> (+1.13%)	⬆️
core/sdk/src/clients/producer_dispatcher.rs	95.30% <91.02%> (+0.70%)	⬆️

... and 40 files with indirect coverage changes

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