test: add Stock value object unit tests

[varnika-dev]

What this PR adds

Added first unit tests for the Stock value object in the Catalogs service.

Tests Added (4 tests)

1. Should_Create_Stock_When_All_Values_Are_Valid
   → Stock.Of(10, 5, 100) creates successfully with correct values

2. Should_Throw_When_Available_Exceeds_MaxStockThreshold
   → Stock.Of(600, 5, 100) throws MaxStockThresholdReachedException

3. Should_Throw_When_Available_Is_Zero
   → Stock.Of(0, 5, 100) throws zero stock not allowed

4. Should_Throw_When_RestockThreshold_Is_Zero
   → Stock.Of(10, 0, 100) throws Zero restock threshold not allowed

Also Fixed

• global.json SDK version updated from 9.0.303 to 9.0.115
  to match locally installed .NET SDK

Why Stock

Stock is the most complex value object in the domain
it owns 3 values and enforces a cross-field business rule
(Available cannot exceed MaxStockThreshold).
It is the highest value target for unit testing.