Report DirFileSystem async support from its backend - #2213

DirFileSystem inherits async_impl=True from AsyncFileSystem even when it wraps a synchronous filesystem. That makes instance capability checks report support the wrapped filesystem does not provide.

Set async_impl on each DirFileSystem instance from its backend, following the direction discussed in #1709. This leaves asynchronous context validation unchanged and keeps True for async backends, including those used through their synchronous interface.

Regression tests cover memory, local and nested directory backends, plus AsyncFileSystemWrapper over memory in both sync and async contexts. They also exercise file reads and writes. Add a changelog entry.

Addresses #1709. This does not change cache filesystem behavior or revise the broader async documentation questions in that issue.

Validation:

Fresh clone with an editable install on Python 3.12:

• Baseline regressions: 3 failed, 2 passed (memory/local/nested dir falsely report async support).
• Patched DirFileSystem, async wrapper and cache suites: 309 passed, 32 skipped.
• Ruff 0.14.3 check and format checks pass. git diff --check passes.
• Broader downstream/service integration tests not run; repository guidance says backend-focused tests suffice locally. Skips reflect optional backend dependencies.