Delegate local file capabilities to the underlying handle

[PDGGK]

Problem

A local text update opened through fsspec.open(path, "r+", encoding="utf-8") can raise io.UnsupportedOperation: not writable, although the underlying r+b file supports writes. The filesystem("file").open route has the same result. Write-only local streams also report readable=True.

The capability methods introduced in #589 use mode-string heuristics. TextIOWrapper relies on those reported capabilities, so an incorrect result prevents otherwise supported updates.

Change

Delegate LocalFileOpener.readable() and writable() to the underlying file object, as the other I/O operations already do. Add capability, public text-update, closed-handle, pickle and compression regression tests, plus a changelog entry.

The change is limited to these two capability expressions. It does not change mode parsing, stream ownership, transactions or remote filesystems, or add support for compressed update modes.

Validation

Tested locally on Linux with Python 3.12.14 only:

• Local/core/spec/compression/API/cached/dirfs/memory suites: 896 passed, 70 skipped, 2 xfailed.
• Final regression selection: 20 passed. Reverting only production code gives 13 failures and 7 passes with the same tests.
• An additional 45-case check covers public opener routes, binary/text modes, persisted bytes against builtin open, pickle, closing behavior, and gzip/bz2/xz.
• Applicable changed-file pre-commit hooks and whitespace checks pass.

The skips include unavailable optional dependencies and Windows-specific checks. The full upstream Python/OS matrix, remote integrations and downstream CI were not run locally. A multiprocessing/fork deprecation warning remains in the tested suite.