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
• /request-review @user-or-team - request a reviewer

See CONTRIBUTING.md for details.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.31%. Comparing base (c0c7493) to head (635eed5).

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3933       +/-   ##
=============================================
- Coverage     83.87%   70.31%   -13.56%     
  Complexity     1358     1358               
=============================================
  Files          1212     1212               
  Lines        166843   142867    -23976     
  Branches     134306   110454    -23852     
=============================================
- Hits         139937   100459    -39478     
- Misses        23266    38588    +15322     
- Partials       3640     3820      +180     

Components	Coverage Δ
Rust Core	67.07% <100.00%> (-17.58%)	⬇️
Java SDK	66.67% <ø> (ø)
C# SDK	75.06% <ø> (-1.49%)	⬇️
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	95.84% <ø> (ø)
Go SDK	68.32% <ø> (ø)

Files with missing lines	Coverage Δ
core/sdk/src/clients/producer_sharding.rs	95.85% <100.00%> (+1.18%)	⬆️

... and 354 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.