Add comprehensive validation tests for MultiQueryExpander

[alxkm]

This PR adds additional test coverage for the MultiQueryExpander builder validation logic.

Changes

• Added tests for valid prompt template configurations with both required placeholders
• Added tests for prompt templates containing extra placeholders beyond the required ones
• Added tests for builder behavior when properties are set multiple times
• Added tests for null prompt template handling after setting a valid template
• Added tests for case-sensitive placeholder validation

Test Coverage

These tests complement the existing validation tests by covering:

• Positive cases: valid templates that should successfully build
• Edge cases: templates with extra placeholders, builder property overrides
• Negative cases: case-sensitivity of placeholder names

Validation

All tests follow the existing test patterns and use consistent assertions with the current test suite.

[ilayaperumalg]

@alxkm Thanks for the PR and @sunyuhan1998 thanks for the review.