Tests: Add unit tests for core types (string, optional, unique_ptr, delegate, Formatter)

[io-eric]

What: Add a test harness and unit tests covering copy/move semantics and basic correctness for core utilities.

Where: New tests/ directory; tests should cover files under include/webcc/core/

Proposed fix:

• Add tests for string (construct/copy/move/equality), string_view equality, optional construct/reset/destruct, unique_ptr move/release/reset, delegate invocation, and Formatter formatting.
• Add a simple test runner or integrate a small test framework and hook it into CI.

Acceptance criteria:

• Tests are added and pass locally.
• CI runs the tests on PRs and main.