feat(format): read the other attributes when one cannot be read

attrs() read every attribute of an object through extract_attributes_full,
so one attribute it could not read (a corrupt or unsupported attribute
message, or a heap object it could not locate) failed all of them — the
same shape as the huge-object bug, where one 8 KiB attribute hid every
attribute on a NetCDF file's root group.

- clawhdf5-format: new attribute::extract_attributes_tolerant returns the
  attributes it could read plus one error per attribute it could not.
  Errors in the attribute index itself (Attribute Info message, dense
  heap header, B-tree) still fail, since then it is unknown which
  attributes exist. extract_attributes_full is unchanged (strict); both
  share one implementation.
- clawhdf5: attrs() on Group/Dataset, MmapGroup/MmapDataset and
  LazyGroup/LazyDataset leaves an unreadable attribute out (documented),
  and the new attrs_with_errors() returns the map with the per-attribute
  errors. A value is either returned complete or not at all.

Regression test: one_unreadable_attribute_does_not_hide_the_others (h5py
writes 11 dense attributes; one message's version byte is corrupted;
before: attrs() failed with InvalidAttributeVersion(127), after: the 10
others come back with their values and one error is reported).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/known-issues.md

@@ -87,7 +87,9 @@ the VDS item, which is marked.
 - **Dense attributes:** a large attribute stored as a fractal-heap "huge"
   object makes every attribute on the object fail. This affects real NetCDF
   files (`issue671.nc`). **Fixed 2026-09-25:** huge and tiny heap objects,
-  and filtered heaps, are read.
+  and filtered heaps, are read; and an attribute that still cannot be read
+  is left out of `attrs()` (reported by `attrs_with_errors()`) instead of
+  failing the others.
 - **Other readers:**
   - VL-string datasets are not readable through `File`.
   - Metadata cache images are not supported.