Added comprehensive unit test suite for Mixin sink

[TanmayRanaware]

fixes #32

• Created BDD-style test suite following existing patterns
• Tests cover plain objects, futures/promises, event listeners
• Includes edge cases and complex scenarios
• Validates sink configuration structure
• Ensures proper attribute application and removal

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[dariomannu]

Really good so far, just left a couple of notes, if you could please review them...

[dariomannu]

@TanmayRanaware:

8 tests failed:
✗ Mixin Sink > Given a plain object mixin > when applying attributes to element > when boolean attributes are provided > handles boolean attributes correctly [3.00ms]
✗ Mixin Sink > Given a plain object mixin > when applying attributes to element > when falsey values are provided > removes attributes when set to falsey values [2.00ms]
✗ Mixin Sink > Given a future/promise mixin > when applying future attributes > when promise resolves successfully > applies future attributes when promise resolves [18.00ms]
✗ Mixin Sink > Given event listener mixins > when handling future event listeners > when promise resolves with event handlers > handles future event listeners [14.00ms]
✗ Mixin Sink > Given complex mixin scenarios > when handling mixed attribute types in single mixin > when all attribute types are combined > handles mixed attribute types in single mixin [2.00ms]
✗ Mixin Sink > Given edge cases > when an empty mixin object is given > when no attributes are provided > handles empty mixin object
✗ Mixin Sink > Given edge cases > when null or undefined values are given > when attributes have null/undefined values > handles null and undefined values in mixin
✗ Mixin Sink > Given edge cases > when string "false" values are given > when attributes contain string "false" > handles string "false" values correctly [1.00ms]