test: add unit tests for validate() and NewFromMap() functions

[raphaeltorquat0]

Summary

• Adds comprehensive unit tests for validate() and NewFromMap() functions in pkg/util/config/config.go
• Addresses issue Add unit tests to config util functions #416

Changes

• TestValidate: Tests validation logic including:

  • Valid configuration passes validation
  • MinInstances > MaxInstances returns error
  • Workers = 0 returns error
  • ConnectionPooler.NumberOfInstances < minimum returns error
  • ConnectionPooler.User = SuperUsername returns error
  • MinInstances/MaxInstances both negative (disabled) passes validation

• TestNewFromMap: Tests config creation from map including:

  • Empty map uses default values
  • Custom values override defaults
  • Duration parsing (e.g., "10s", "1m")
  • Boolean parsing
  • Map parsing (e.g., "key1:value1,key2:value2")
  • Slice parsing (e.g., "item1,item2,item3")
  • Invalid workers triggers validation panic
  • Invalid integer causes panic

• TestMain: Sets OPERATOR_NAMESPACE environment variable to avoid log.Fatal when running tests outside a Kubernetes pod

Test plan

• All unit tests pass locally (go test ./pkg/util/config/...)
• Full test suite passes (go test ./...)

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Disclosure: I used AI assistance (Claude) to help implement this change under my review and testing.

Closes #416

[raphaeltorquat0]

Hi @FxKu, could you please add the minor label so the CI checks can proceed? Thanks!