Reuse parsed stats in `scan_metadata_from`

[dengsh12]

Problem

scan_metadata_from reshapes cached scan-file metadata into checkpoint-like Add actions, but the transform preserves only JSON stats and raw partitionValues.

At kernel/src/scan/mod.rs:975-978, it uses get_scan_metadata_transform_expr() with restored_add_schema().

get_scan_metadata_transform_expr() (kernel/src/scan/log_replay.rs:923-934) preserves stats at line 928 but omits the optional stats_parsed and partitionValues_parsed columns.

The resulting checkpoint information then unconditionally declares:

has_stats_parsed: false,
has_partition_values_parsed: false,

at kernel/src/scan/mod.rs:1048-1051.

When the current scan needs typed stats or partition values, log replay consequently reparses them:

• Expression::parse_json(...) at kernel/src/scan/log_replay.rs:896
• Expression::map_to_struct(...) at kernel/src/scan/log_replay.rs:909

Cached scan metadata may already contain these parsed columns. In addition to repeated work, dropping stats_parsed can reduce pruning when JSON stats is null.

Proposed optimization

1. Accept the actual cached scan-file schema, including optional parsed columns.
2. Preserve stats_parsed and partitionValues_parsed when reshaping cached rows into Add actions.
3. Verify that their schemas are compatible with the current scan’s physical stats and partition schemas.
4. Build CheckpointReadInfo dynamically:
   • Set the parsed flags when compatible parsed columns are preserved.
   • Include those columns in checkpoint_read_schema.
5. Fall back to parsing JSON stats or mapping raw partition values when parsed columns are absent or incompatible.

Testing

Add coverage where:

• A previous scan emits stats_parsed.
• A previous scan emits partitionValues_parsed.
• scan_metadata_from reuses compatible parsed columns.
• Incompatible parsed schemas fall back safely.
• Cached data with optional parsed columns satisfies evaluator input-schema validation.