Fix off-by-one in app group identifier length check for POSIX semaphore prefix

[taichino]

Summary

On sandboxed macOS, Store.init reads the app's entitlements and picks the first
application group identifier that fits within the POSIX semaphore name budget.
The current filter checks $0.length <= 20 before appending "/", but
obx_posix_sem_prefix_set requires the total prefix (including "/") to be at most
20 characters. A group ID of exactly 20 characters passes the filter, becomes 21
after appending, and is rejected by the C library.

The rejected call sets thread-local error state, and because the return value of
obx_posix_sem_prefix_set is not checked, this stale error propagates into
Store.init via checkLastError(), causing initialization to fail with
illegalArgument("Given prefix must not exceed 20 chars").

This was discovered in a production app whose shortest app group identifier is
exactly 20 characters (team ID + "." + 9-char name).

Changes

• Changed the length filter from <= 20 to <= 19 to account for the appended "/"
• Updated the comment to clarify that the limit applies to the full prefix including "/"

Testing

This is not covered by a unit test because setUpMutexIdentifier reads entitlements
from the running binary's code signature at runtime, which cannot be controlled in a
test environment. The fix is verified by code inspection.

[greenrobot-team]

Thanks for this! We should probably also change this to work like in the Dart API where a missing slash suffix is added before the length check.

For our reference this is internal issue objectbox-swift#351

[taichino]

Thanks for the suggestion! I've updated the PR to match the Dart API approach.
I also extracted the logic into a static func semaphorePrefix(from:) so it's unit testable.

@greenrobot-team

[greenrobot-team]

@taichino I didn't expect you to make these changes (hence the "we" as in the ObjectBox team), so thank you! We will look at picking these changes when there is time!