fix(obslist): post-merge cleanup from PR #394 review (issues #2, #4)

[brickbots]

Two deferred follow-up fixes from the review of #394
(feat: multi-format observing list import with native .pifinder support).
Both are low-risk and independent; neither was a merge blocker.

#2 — Cross-catalog description query comment (ui/object_details.py)

The shared ObjectsDatabase handle was documented as "only used when
cross-catalog descriptions are enabled" — but no such setting exists and
_other_catalog_descriptions() runs unconditionally on the description view.

Resolution: keep the feature always-on and fix the comment (the product
call). The per-object query is a single indexed lookup and returns {} for
virtual planets/comets/OBS coordinate objects, so the cost is negligible.
The comment now also documents the connection lifecycle: the read handle lives
for the UI process and is released at process exit, matching the existing
per-instance ObservationsDatabase in the same file. No behaviour change.

#4 — read_list resolution loop could crash the UI (obslist.py)

read_list only wrapped the file read in try/except; the resolution loop was
unprotected, so a DB error, uninitialised catalogs, or a malformed entry
propagated to UIObsList.key_right (no handler) and crashed the UI instead of
showing the existing "Error loading" message.

• Per-entry guard: one malformed entry is logged and skipped; the rest of
  the list still loads (result stays "success", matched < parsed).
• Systemic failure (every entry raises, e.g. catalog DB unavailable) is
  returned as the error dict instead of a silent "0 objects" success.
• Outer safety net mirrors the file-read error-dict contract for any
  failure outside the per-entry guard, counting len(list_catalog) so it
  can't re-raise on the same broken entries.

Return shape is unchanged, so UIObsList.key_right works without modification.

Tests

Adds TestReadListErrorHandling to tests/test_obslist_resolve.py (unit
marker) covering all three paths: per-entry skip, systemic error, and the
outer net.

pytest -m unit tests/test_obslist_resolve.py tests/test_obslist_formats.py \
               tests/test_list_descriptions.py
# 54 passed

ruff check (with builtins=['_']) and ruff format --check clean on all
changed files.

Out of scope (documented in #394's review thread): #1 real-file fixtures,
#3 session-wide description accumulation (ADR-0011), and nits #5/#6.

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