fix: validate MaxBodyCaptureSizeKb rejects negative values

[DevSars24]

Summary

Fixes #113

MaxBodyCaptureSizeKb could previously be set to negative values, which caused incorrect body size calculations and unexpected capture behavior during request/response truncation.

Root Cause

The property had no validation on the setter, allowing any integer value including negatives.

Changes

DebugProbe.AspNetCore/Options/DebugProbeOptions.cs

• Replaced auto-property with a backing field + validated setter
• Throws ArgumentOutOfRangeException with a clear message if value < 0
• Default value of 32 preserved
• 0 correctly disables body capture

DebugProbe.AspNetCore.Tests/Configuration/DebugProbeOptionsTests.cs

• Added test: setting -1 throws ArgumentOutOfRangeException
• Added test: setting 0 is valid and accepted correctly

Test Results

• 2 new tests added — both passing ✅
• 51 existing tests passing ✅
• 3 pre-existing failures unrelated to this change (net10 PipeWriter compatibility issue)