Allow 0 tokens in a CapacityLimiter instantiated outside an event loop

[nyxst4ck]

Changes

Fixes #.

PR #1019 (released in 4.12) allowed setting a CapacityLimiter's total tokens to 0, updating the docstring, the changelog and the asyncio backend setter (value < 1 → value < 0).

However, the CapacityLimiterAdapter.total_tokens setter in src/anyio/_core/_synchronization.py was missed and still enforced value < 1. This adapter is used whenever a CapacityLimiter is instantiated outside of a running event loop, so the documented 4.12 behavior was not honored on that path:

import anyio

anyio.CapacityLimiter(0)             # ValueError: total_tokens must be >= 1
limiter = anyio.CapacityLimiter(1)
limiter.total_tokens = 0             # ValueError: total_tokens must be >= 1

This aligns the adapter setter with the asyncio backend setter (value < 0, message "total_tokens must be >= 0") and adds a regression test for the outside-event-loop path.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[agronholm]

Looks good, thanks!