Refactor Parser and Parser.Builder

[ani28-bit]

Summary

This PR refactors the parser implementation to improve readability and maintainability without changing functionality.

Changes

• Refactored the duplicated parsing workflow in Parser.
• Extracted extension handling, validation, and inline parser factory logic from Parser.Builder.

Verification

• All existing tests pass successfully with mvn test.
• No external behavior or API changes were introduced.

[robinst]

Thanks. I like some of these changes but don't think others are an improvement at all; closing this.