Add case-heavy LEFT JOIN benchmark and debug timing/logging for PushDownFilter hot paths (#20664)

## Which issue does this PR close?

* Part of #20002.

## Rationale for this change

The `PushDownFilter` optimizer rule shows a severe planner-time
performance pathology in the `sql_planner_extended` benchmark, where
profiling indicates it dominates total planning CPU time and repeatedly
recomputes expression types.

This PR adds a deterministic, CASE-heavy LEFT JOIN benchmark to reliably
reproduce the worst-case behavior and introduces lightweight debug-only
timing + counters inside `push_down_filter` to make it easier to
pinpoint expensive sub-sections (e.g. predicate simplification and join
predicate inference) during profiling.

## What changes are included in this PR?

* **Benchmark: add a deterministic CASE-heavy LEFT JOIN workload**

* Adds `build_case_heavy_left_join_query` and helpers to construct a
CASE-nested predicate chain over a `LEFT JOIN`.
* Adds a new benchmark `logical_plan_optimize_case_heavy_left_join` to
stress planning/optimization time.
* Adds an A/B benchmark group `push_down_filter_case_heavy_left_join_ab`
that sweeps predicate counts and CASE depth, comparing:

    * default optimizer with `push_down_filter` enabled
    * optimizer with `push_down_filter` removed

* **Optimizer instrumentation (debug-only)**

* Adds a small `with_debug_timing` helper gated by `log_enabled!(Debug)`
to record microsecond timings for specific sections.
  * Instruments and logs:

    * time spent in `infer_join_predicates`
    * time spent in `simplify_predicates`
* counts of parent predicates, `on_filters`, inferred join predicates
    * before/after predicate counts for simplification

## Are these changes tested?

* No new unit/integration tests were added because this PR is focused on
**benchmarking and debug-only instrumentation** rather than changing
optimizer semantics.
* Coverage is provided by:

  * compiling/running the `sql_planner_extended` benchmark
* validating both benchmark variants (with/without `push_down_filter`)
produce optimized plans without errors
* enabling `RUST_LOG=debug` to confirm timing sections and counters emit
as expected

## Are there any user-facing changes?

* No user-facing behavior changes.
* The optimizer logic is unchanged; only **debug logging** is added
(emits only when `RUST_LOG` enables Debug for the relevant modules).
* Benchmark suite additions only affect developers running benches.

## LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated
content has been manually reviewed and tested.