fix: support single string and callable in default_limits (#239)

[agustin18]

When initializing Limiter with default_limits="1/second" (a string) instead of a list ["1/second"], set(default_limits) iterated over individual characters of the string ('1', '/', 's', ...), causing LimitGroup to fail parsing the rate limit with a ValueError during request handling. Similarly, passing a callable raised TypeError: 'function' object is not iterable.

Changes

• Add _normalize_limits helper to coerce single string, bytes, callable, or sequence into a list.
• Apply normalization to default_limits, application_limits, and in_memory_fallback in Limiter.__init__.
• Update type annotations to Union[Sequence[StrOrCallableStr], StrOrCallableStr].
• Add parameterized unit tests covering single strings and callables for default limits, application limits, and in-memory fallback in both Starlette and FastAPI test suites.
• Update CHANGELOG.md.

Fixes #239