refactor(parquet-datasource): split bloom_filter out of row_group_filter.rs (apache#22348)

## Which issue does this PR close?

Relates to the discussion in apache#22024 about the Parquet datasource crate
becoming hard to navigate. Split out of apache#22156, which bundled several
code-motion moves into one PR — this is one of three smaller,
independently-reviewable PRs that replace it.

## Rationale for this change

`row_group_filter.rs` had grown to ~1,900 LOC. It mixes "data we loaded
from the file" with "the access-plan filter that consumes it." This PR
is **code motion only**: no behavior change and no public API change.

## What changes are included in this PR?

Extracts `BloomFilterStatistics` — the loaded Split Block Bloom Filter
(SBBF) data plus its `PruningStatistics` adapter — from
`row_group_filter.rs` into a new `bloom_filter.rs`, and moves the
bloom-filter tests alongside it. This separates `BloomFilterStatistics`
(data loaded from the file) from `RowGroupAccessPlanFilter` (the
access-plan filter that consumes it), leaving `row_group_filter.rs`
focused on the latter.

Each commit builds green on its own:

1. **Split `bloom_filter` out of `row_group_filter.rs`** — move
`BloomFilterStatistics` and its `PruningStatistics` adapter into a new
`bloom_filter.rs`.
2. **Build `BloomFilterStatistics` via its constructors in tests** — the
test built it with a struct literal; use the existing
`with_capacity`/`insert` constructors (the pattern `opener.rs` already
uses) so the `column_sbbf` field stays private.
3. **Extract `ExpectedPruning` into a shared `test_util` module** — the
one test helper shared between the row-group and bloom-filter tests, so
the bloom-filter tests can move out. Adds a `#[cfg(test)] pub(crate) fn
access_plan()` accessor on `RowGroupAccessPlanFilter` so the helper can
assert from a sibling module without widening field visibility.
4. **Move the bloom-filter tests into `bloom_filter.rs`** — relocate
`test_row_group_bloom_*`, the `BloomFilterTest` builder, and its helper
next to the code they exercise.

`BloomFilterStatistics` is crate-internal; `row_group_filter` re-exports
it (`pub(crate) use`) so the existing
`crate::row_group_filter::BloomFilterStatistics` path keeps resolving
for in-crate callers. Aside from `row_group_filter.rs` and
`bloom_filter.rs`, this adds a new `src/test_util.rs` and a one-line
module declaration in `mod.rs`.

## Are these changes tested?

Yes, covered by existing tests. `cargo test -p
datafusion-datasource-parquet --all-features` (122 passing) and `cargo
clippy -p datafusion-datasource-parquet --all-targets --all-features --
-D warnings` both pass.

## Are there any user-facing changes?

No. `BloomFilterStatistics` is crate-internal; this only reorganizes
files inside the crate.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb