fix: require callable iterators in iterable assertions

[20ns]

expect({ [Symbol.iterator]: true }).to.be.iterable and assert.isIterable(...) currently pass, even though the value cannot be iterated. Passing it to .members() then throws a native TypeError from Array.from instead of an AssertionError.

Require Symbol.iterator to be a function in both iterable checks. This also makes .not.iterable accept these non-iterable objects and lets .members() reject either invalid input through its existing assertion guard.

Regression coverage includes truthy non-function iterator properties, negation, both member-comparison inputs, custom messages, a generator iterator, and checking iterability without calling the method. The six affected test cases fail against unchanged main and pass with the fix.

Validation:

• pnpm run build
• pnpm test: lint and formatting, 512 Node tests, 511 Chromium tests
• pnpm run test-chrome --browsers firefox: Firefox could not start on this macOS 27.0 host. A standalone Playwright Firefox launch also times out before loading any test code.
• pnpm run lint:types still fails with the same 601 existing diagnostics as unchanged main, after normalizing source positions.

AI assistance: Tested using Codex.