Apply default MaxDataSize when config value is zero

[jhrozek]

Related to PR #3060 where I forgot to set the audit max size, let's default to a sane value rather than the Go default value for the audit max size.

When max_data_size is not specified in the audit config, Go's zero value (0) was used directly. This caused the size check len(body) <= 0 to always fail for non-empty bodies, preventing data capture.

Changes:

• Validate() now applies the default value (1024) when MaxDataSize is 0
• CreateMiddleware() now calls Validate() to ensure defaults are applied
• Added test coverage for default application and value preservation

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.51%. Comparing base (589ecb5) to head (fea0e09).

Files with missing lines	Patch %	Lines
pkg/audit/middleware.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3061      +/-   ##
==========================================
- Coverage   56.51%   56.51%   -0.01%     
==========================================
  Files         334      334              
  Lines       33170    33174       +4     
==========================================
+ Hits        18747    18749       +2     
- Misses      12839    12840       +1     
- Partials     1584     1585       +1     

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