Fix the path cache test fakes and cover the listing sizes they missed

[kandrosov]

Problem

test/test_path_cache.py fails on main — 9 of its 10 tests:

AttributeError: 'Entry' object has no attribute 'is_dir'                     RunKit/law_gfal.py:511
AttributeError: 'FakeGFALFileInterface' object has no attribute 'listing_sizes'  RunKit/law_gfal.py:508

#307 added listdir_info, _cache_listing_sizes and self.listing_sizes to
GFALFileInterface, reading entry.is_dir and entry.size off a listing. The test file, added
in #304, models a listing entry as namedtuple("Entry", ["name"]) and builds its fake interface
by assigning voms_token / path_cache / verbose by hand instead of calling the real
__init__ — so neither the new attribute nor the new fields existed.

Confirmed to predate the current main: the suite already fails at 59daf8d (#307), before
#306 merged. The production code is correct; only the test doubles were stale. Nothing runs this
suite in CI, which is why it went unnoticed.

Solution

• FakeGFALFileInterface now calls the real __init__ with only the grid-proxy lookup
  stubbed out, rather than mirroring the attributes it sets. That is what makes the fake track
  GFALFileInterface from now on: hand-mirroring is precisely what broke when listing_sizes
  appeared, and it broke silently until something read it.
• Entry carries the fields of RunKit.grid_tools.FileInfo that law_gfal actually reads —
  name, size, is_dir — with defaults so the existing cases stay readable.
• The fake tree may now map a directory to {name: size} as well as to a list of names, and an
  entry that is itself a key of the tree is reported as a directory, as a real listing would.

Also: the listing sizes had no test

listdir_info is the reason the fakes broke and had no coverage of its own. Three tests added:

• sizes come from the listing and directories are excluded;
• asking for sizes after an exists() that already listed the directory costs no second
  gfal-ls — the actual claim in its docstring;
• a failed listing yields {} rather than raising.

Testing

• test/test_path_cache.py: 13 passed (was 1 passed / 9 failed).
• Whole test/ directory with ANALYSIS_PATH set: 79 passed.
• The new tests were checked by mutation, so they are not vacuous:
  • dropping the not entry.is_dir filter in _cache_listing_sizes → the exclusion test fails;
  • dropping the _cache_listing_sizes call in listdir → the reuse test fails.
    law_gfal.py was restored unmodified afterwards.

No docs change: internal only — test doubles and test coverage, no user-visible behaviour.