Translate unsupported fsspec modification times into a typed DuckDB error

Start at PythonFilesystem::GetLastModifiedTime() and trace how DirectFileReader handles optional metadata errors. Add a regression test using a custom fsspec filesystem with unsupported modified() behavior, then run it through read_blob(...).last_modified. Done means unsupported modification times return NULL while unrelated filesystem errors still surface.

What happens?

When a filesystem registered through Connection.register_filesystem() cannot provide a modification time, querying the last_modified column of read_blob surfaces the raw Python exception instead of returning NULL.

One concrete case is gcsfs: GCSFileSystem.modified() indexes info(path)["mtime"], while some GCS object metadata (and synthesized directory entries) has no mtime. The resulting KeyError: 'mtime' currently reaches a DuckDB query as an untyped error and can abort callers such as DuckLake CHECKPOINT.

This is the Python/fsspec half of:

• duckdb/ducklake#1042
• duckdb/ducklake#1336

Why the error loses its type

PythonFilesystem::GetLastModifiedTime() currently calls filesystem.modified() directly. A Python exception crosses the nanobind boundary as std::exception; when DuckDB later constructs ErrorData(ex), it has ExceptionType::INVALID, so extensions cannot distinguish unsupported modification-time metadata from unrelated I/O failures without inspecting the message.

DuckDB core already has the desired downstream behavior: DirectFileReader converts ExceptionType::NOT_IMPLEMENTED failures for optional file metadata columns to NULL. An older_than predicate then naturally retains only files whose age is unknown.

Suggested direction

Translate the Python filesystem capability error at the adapter boundary in PythonFilesystem::GetLastModifiedTime():

1. Catch nb::python_error.
2. Map Python NotImplementedError to DuckDB NotImplementedException.
3. Consider a narrowly scoped compatibility mapping for the current gcsfs KeyError('mtime'), or coordinate with gcsfs so missing mtime is reported as NotImplementedError.
4. Re-throw every unrelated Python exception unchanged.

Please add a regression test with a small custom fsspec filesystem whose modified() is unsupported, asserting that read_blob(...).last_modified is NULL while other filesystem errors still surface.

This keeps provider-specific exception classification in the Python filesystem adapter instead of requiring downstream extensions to accumulate message-matching helpers.