fix(parquet): exclude single-leaf struct roots from predicate cache

[imhy]

Which issue does this PR close?

• Closes parquet predicate-cache: panic / silent row drop on single-leaf nullable struct #9982 .

Rationale for this change

What changes are included in this PR?

Root cause

ProjectionMask::without_nested_types (parquet/src/arrow/mod.rs:427) decides which leaves the predicate cache may cover. The check before this fix was:

if root_leaf_counts[root_idx] == 1 && !root.is_list() {
    included_leaves.push(leaf_idx);
}

PR #8866 added !root.is_list() to exclude lists, but a struct root with a single leaf still satisfies the condition and gets cached.

Fix (1 line)

parquet/src/arrow/mod.rs:455:

-                if root_leaf_counts[root_idx] == 1 && !root.is_list() {
+                if root_leaf_counts[root_idx] == 1 && root.is_primitive() {
                     included_leaves.push(leaf_idx);
                 }

Are these changes tested?

Tests added on the branch

1. Reproducer integration test

File: parquet/tests/arrow_reader/predicate_cache.rs
Name: test_async_predicate_on_single_leaf_nullable_struct

Builds an in-memory Parquet file with OPTIONAL group b { REQUIRED BYTE_ARRAY aa (UTF8); }, writes two rows (parent NULL, parent non-NULL), then runs the same IS NULL row filter through the async reader twice: once with the default cache, once with with_max_predicate_cache_size(0). It asserts that

• the uncached control yields exactly 1 row (address NULL row matches);
• the cached run yields the same row count as the uncached one.

Pre-fix: panic at struct_array.rs:142.
Post-fix: passes (1 row in both cases).

2. Unit test

File: parquet/src/arrow/mod.rs (test module)
Name: test_projection_mask_without_nested_single_leaf_struct

Directly checks ProjectionMask::without_nested_types against a schema with OPTIONAL group address { REQUIRED BYTE_ARRAY street; } REQUIRED INT32 id, for three input masks (single nested leaf, mixed, all leaves). All three expected outputs reflect that the struct's leaf is now considered nested.

Pre-fix: would return Some([street_leaf]) for the single-leaf-only mask.
Post-fix: returns None for the single-leaf-only mask; returns Some([id]) for mixed.

Verification matrix

Test	Pre-fix	Post-fix
test_projection_mask_without_nested_single_leaf_struct (new unit)	would FAIL	PASS
test_async_predicate_on_single_leaf_nullable_struct (new integration)	PANIC	PASS
predicate_cache::test_default_read	PASS	PASS
predicate_cache::test_async_cache_with_filters	PASS	PASS
predicate_cache::test_sync_cache_with_filters	PASS	PASS
predicate_cache::test_cache_disabled_with_filters	PASS	PASS
predicate_cache::test_cache_projection_excludes_nested_columns	PASS	PASS
test_projection_mask_without_nested_* (5 existing)	PASS	PASS

Are there any user-facing changes?