[Follow-up #23696] Prebuild parquet row-filter candidates once per file

[zhuqi-lucas]

Follow-up to #23696, per @adriangb's decomposition proposal (PR 2).

RowFilterGenerator::build() currently redoes split_conjunction + FilterCandidateBuilder::build + reassign_expr_columns for every row group. On any multi-row-group scan with a pushdown predicate, that per-RG tree walk and column resolution is wasted work.

Move it to once per file: prebuild the row-filter candidate list when the file is opened, then instantiate a RowFilter per row group cheaply from the prebuilt list.

Proposed shape (extracted from #23696, decoupled from fully_matched):

• add PrebuiltRowFilterCandidate / prebuild_row_filter_candidates (once per file: tree walk + candidate construction) and row_filter_from_prebuilt (per RG: cheap bind against the fresh array readers);
• delete RowFilterGenerator;
• reimplement the existing public build_row_filter on top of the two new functions (its signature must stay — parquet_nested_filter_pushdown / parquet_struct_filter_pushdown benches depend on it).

Parity to preserve: same conjunct order (sort_unstable_by_key(required_bytes) when reorder_predicates), same metric wiring (every predicate shares pushdown_rows_pruned, only the last predicate counts pushdown_rows_matched). One intended behavioural delta: reassign_expr_columns errors surface once at open time instead of being swallowed per-build by log::debug!.

Needs nothing from fully_matched.