PR Description:
This PR enhances test coverage for core message classes by adding comprehensive unit tests covering edge cases and validation scenarios.
Changes:

• SystemMessage: Added 10 new tests covering empty/whitespace text, builder validation, metadata handling, immutability, mutation chains, and object contracts
• UserMessage: Added 14 new tests covering similar scenarios plus media handling, copy operations, and toString methods
• MessageUtils: Added 1 additional validation test for null resource with charset parameter

Test Coverage Added:

• Edge cases (empty text, whitespace, null inputs)
• Builder pattern validation and metadata overwriting
• Copy/mutation immutability guarantees
• Object contract compliance (equals, hashCode, toString)
• Media attachment handling for UserMessage