test: Enhance PostgresMlEmbeddingOptions test coverage

[alxkm]

Description

This PR enhances test coverage for PostgresMlEmbeddingOptions by adding tests for previously untested methods and documenting important builder behavior.

Changes

• Added 6 new test cases covering setters, interface methods, and builder behavior
• Documents that builder reuses the same instance (not creating new ones on each build())
• Validates edge cases and null handling

Test Coverage Added

• Setter functionality - Validates setters work directly on options instance
• Interface compliance - Tests getModel() and getDimensions() return null as designed
• Builder behavior - Documents that builder returns same instance on multiple build() calls
• Builder mutation - Shows that modifying builder after build affects previous instances
• Null handling - Verifies kwargs setter accepts null values

Value

The tests reveal that the builder pattern implementation reuses the same options instance rather than creating new instances on each build() call. This behavior is now explicitly tested and documented, which is important for developers to understand as it differs from typical builder pattern expectations.

[ilayaperumalg]

@alxkm Thanks for the PR and @sunyuhan1998 Thanks for the review. Rebased and merged as 838801f