test: add test coverage for SyncMcpToolCallback.Builder

[alxkm]

Description
This PR adds comprehensive test coverage for the SyncMcpToolCallback.Builder class, focusing on tool name handling, validation, and builder behavior.

Changes
Added 5 new test methods to SyncMcpToolCallbackBuilderTest:

• Tool name normalization: Verifies that tool names with dashes are normalized to underscores
• Custom prefix bypass: Ensures custom prefixed names are used as-is without normalization
• Null prefix handling: Tests that null prefixed names fall back to normalized defaults
• Instance creation: Verifies the builder creates new instances for each build() call
• Converter validation: Ensures null converters are properly rejected with appropriate error message

Impact
The existing tests only covered basic builder functionality. These new tests add coverage for edge cases and special behaviors around tool name normalization, custom naming, and proper validation of required fields, ensuring the builder behaves correctly in all scenarios.

[ilayaperumalg]

@alxkm Thanks for the PR! Rebased and merged as 46b9170