chore(deps): update actions/setup-node action to v6

…ific messaging

Convert AssertError and AssertNoError from derived functions that delegate to
nil assertions into independent base functions with error-specific messaging.

Changes:
- AssertError now directly checks err != nil and produces "expected error, got nil".
- AssertNoError now directly checks err == nil and produces "unexpected error: %v".
- Both functions log domain-appropriate success messages.
- Update TESTING_core.md to reflect the new architecture hierarchy.
- Maintain 100% test coverage and all existing tests pass.

This improves clarity when debugging test failures by providing error-oriented
messages rather than generic nil-checking messages.

Signed-off-by: Alejandro Mery <amery@apptly.co>

Add negative variants of string containment assertions to complement the
existing AssertContains functionality.

Implementation:
- Add AssertNotContain to verify substring absence.
- Add AssertMustNotContain for fatal variant that calls FailNow.
- Both functions follow established patterns with appropriate error messages.
- Use "NotContain" (not "NotContains") for grammatical consistency.

Testing:
- Add comprehensive unit test for AssertNotContain.
- Add integration test for AssertMustNotContain with MockT.
- Achieve 100% test coverage for both new functions.
- Overall package coverage improved to 97.4%.

Documentation:
- Update TESTING_core.md function hierarchy.
- Add examples to TESTING.md Advanced Assertions.
- Update README.md with new assertions in both basic and fatal lists.
- All documentation maintains consistent formatting and passes linting.

The implementation provides clear error messages:
- Success: "does not contain %q".
- Failure: "expected %q not to contain %q".

Signed-off-by: Alejandro Mery <amery@apptly.co>

This commit addresses multiple documentation issues identified during API
verification, ensuring all function signatures and descriptions accurately
reflect the actual implementation.

Key fixes:
- Fix AddrPort function signature from AddrPort(addr, port) to AddrPort(v)
  and correct description to reflect extraction rather than creation.
- Correct Zero function signature from Zero[T]() to Zero[T](_ *T) to show
  the required dummy parameter for type inference.
- Update all assertion function signatures to use 'name...' instead of
  'msg...' to accurately reflect actual parameter names (affects 25+
  functions including AssertEqual, AssertTrue, AssertMust* variants).
- Fix IsError function signatures by removing incorrect generic type
  parameters [T] and updating parameter lists to match implementation.

The documentation now provides 100% accurate function signatures that
match the actual Go implementation, improving developer experience and
reducing confusion when using the API.

Signed-off-by: Alejandro Mery <amery@apptly.co>

Rename AGENT.md to AGENTS.md as AGENTS.md is now the standard.
Update all internal references within the renamed file and
cross-references in README.md to maintain documentation integrity.

Signed-off-by: Alejandro Mery <amery@apptly.co>

Testing Infrastructure Enhancement and Documentation Overhaul