fix: normalize set_indexing mode (lowercase key) and empty target ([]) for Data API

[erichare]

Summary

CollectionDefinition.set_indexing had two related normalization bugs that produced an
indexing payload the Data API does not expect. CollectionDefinition.as_dict() forwards
the indexing value verbatim (only outer None/{} entries are filtered), so a malformed
dict reaches the wire. The Data API expects {"allow"|"deny": [<paths>]} — a lowercase
mode key mapping to a list of paths (as the attribute's own docstring states).

Bug 1 — target not normalized (value)

It computes a normalized local _i_target = indexing_target or [], but then builds the
returned definition from the raw indexing_target:

_i_target: list[str] = indexing_target or []
return CollectionDefinition(
    ...
    indexing={indexing_mode: indexing_target},   # raw param, not _i_target
    ...
)

So _i_target was dead code, and set_indexing("deny") / set_indexing("allow") with no
target stored {mode: None} instead of the intended {mode: []}.

Bug 2 — mode not normalized (key)

The mode is validated case-insensitively (_i_mode = indexing_mode.lower(), checked
against INDEXING_ALLOWED_MODES = {"allow", "deny"}), but the dict is then keyed by the
raw indexing_mode:

_i_mode = indexing_mode.lower()
if _i_mode not in INDEXING_ALLOWED_MODES: ...
return CollectionDefinition(
    ...
    indexing={indexing_mode: ...},   # raw param, not _i_mode
    ...
)

So set_indexing("DENY") / set_indexing("Allow") pass validation but store
{"DENY": ...} / {"Allow": ...} instead of the lowercase {"deny": ...} / {"allow": ...}
the Data API expects.

Fix

Construct the definition from the already-computed normalized locals:
indexing={_i_mode: _i_target}. Both fixes simply activate normalization that was already
written (.lower() and or []) but never wired into the returned object.

Behavior

Before:

set_indexing("deny")  -> {'deny': None}
set_indexing("DENY")  -> {'DENY': []}    # after the target fix: still a wrong-cased key

After:

set_indexing("deny")  -> {'deny': []}
set_indexing("DENY")  -> {'deny': []}

Explicitly-provided targets are preserved; only their normalization (key case, empty value)
changes.

Notes

• Both are pre-existing bugs tracing to the original implementation; not introduced by any
  open PR. Same class of latent "normalize-then-discard" defect.
• Scope deliberately minimal: only the set_indexing constructor arguments change.

Test plan

• test_set_indexing_empty_target_normalization — {"deny": []} / {"allow": []} for
  both .indexing and .as_dict(), plus an explicit-target guard.
• test_set_indexing_mode_case_normalization — "DENY" / "Allow" normalize to
  lowercase keys for both .indexing and .as_dict(), including with an explicit target.
• uv run pytest tests/base/unit/test_collection_options.py — 4 passed
• ruff check + ruff format --check (astrapy + tests) — clean
• mypy astrapy tests — no issues found (214 source files)
• CHANGES updated under the main section (covers both normalizations)