fix(csharp): set default socket buffer size to OS default

[richardcocks]

Closes #4010

Rationale

The previous default buffer size was too small, affecting throughput.

What changed?

A null value for the socket buffer will cause the OS to auto-size the buffer, rather than being a fixed small buffer. Configuration and Builder validation now accepts null, but still validates for positive integers when set.

AI Usage

AI was used to quickly scaffold benchmarks, discover and diagnose the issue, as well as quickly validate the fix-shape, but all code in this PR was hand-edited.

[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

❌ Patch coverage is 73.33333% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.97%. Comparing base (831b2a6) to head (b06ff7b).

Files with missing lines	Patch %	Lines
...SDK/IggyClient/Implementations/TcpMessageStream.cs	0.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4013      +/-   ##
============================================
- Coverage     85.13%   84.97%   -0.17%     
  Complexity     1402     1402              
============================================
  Files          1226     1225       -1     
  Lines        181308   179238    -2070     
  Branches     147612   145532    -2080     
============================================
- Hits         154359   152307    -2052     
- Misses        22902    22933      +31     
+ Partials       4047     3998      -49     

Components	Coverage Δ
Rust Core	85.87% <ø> (-0.20%)	⬇️
Java SDK	67.29% <ø> (ø)
C# SDK	75.45% <73.33%> (+0.07%)	⬆️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.24% <ø> (ø)
Go SDK	69.31% <ø> (ø)

Files with missing lines	Coverage Δ
...p/Iggy_SDK/Configuration/IggyClientConfigurator.cs	100.00% <100.00%> (ø)
...n/csharp/Iggy_SDK/Consumers/IggyConsumerBuilder.cs	81.76% <100.00%> (+3.52%)	⬆️
...gn/csharp/Iggy_SDK/Consumers/IggyConsumerConfig.cs	100.00% <100.00%> (ø)
...reign/csharp/Iggy_SDK/Factory/IggyClientFactory.cs	95.00% <100.00%> (+1.25%)	⬆️
...csharp/Iggy_SDK/Publishers/IggyPublisherBuilder.cs	50.93% <100.00%> (+2.80%)	⬆️
.../csharp/Iggy_SDK/Publishers/IggyPublisherConfig.cs	100.00% <100.00%> (ø)
...SDK/IggyClient/Implementations/TcpMessageStream.cs	80.26% <0.00%> (-0.82%)	⬇️

... and 75 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.

[hubcio]

foreign/csharp/Benchmarks/Program.cs still pins int.MaxValue per OS. worth re-measuring against the new default and dropping the override if it no longer buys anything.

[spetz]

Thank you for the contribution, I pushed a few more fixes, so that we could merge this before the upcoming release :)