feat: run hll_sketch_estimate natively by default and narrow HLL incompatibility reasons - #7550

Which issue does this PR close?

Closes #7396.

Rationale for this change

All four HLL expressions required allowIncompatible because "estimates may differ slightly". That reason was written against datasketches 0.3.0. After the upgrade to 0.6.0 (#7395), I compared Comet's code paths with the DataSketches-Java 6.2.0 logic Spark uses (experiment run, results in #7396 (comment) and #7396 (comment)):

So hll_sketch_estimate can run natively by default, while the other three still produce bytes that differ from Spark and stay Incompatible, now with a reason that says exactly what differs.

Note: the experiment mirrors Spark's Java code and Comet's accumulator logic outside a real Spark/DataFusion plan. The tests in this PR verify end to end in CI.

What changes are included in this PR?

• CometHllSketchEstimate: now Compatible (no allowIncompatible needed on Spark 4.0+).
• CometHllSketchAgg, CometHllUnion, CometHllUnionAgg: still Incompatible, with reasons replaced by the concrete differences above. Shared text lives in a new CometHllReasons.
• The error class difference (Comet execution error vs Spark's HLL_INVALID_INPUT_SKETCH_BUFFER / HLL_UNION_DIFFERENT_LG_K) moves from getIncompatibleReasons to getCompatibleNotes. Both engines fail on the same inputs. The old reason also listed the wrong SQLSTATE for HLL_INVALID_INPUT_SKETCH_BUFFER (22546, not 22000).
• Docs: expressions.md updated, and audit entries added to expression-audits/agg_funcs.md and misc_funcs.md.

Follow-ups filed by this audit:

• [Bug] hll_sketch_agg, hll_union and hll_union_agg write compact sketch bytes where Spark writes updatable ones #7548 Comet writes LIST/SET mode sketches in the compact layout instead of Spark's updatable layout.
• [Bug] hll_* errors surface as Comet errors instead of HLL_INVALID_INPUT_SKETCH_BUFFER / HLL_UNION_DIFFERENT_LG_K #7549 HLL errors do not use Spark's error classes.

How are these changes tested?

• New SQL file test expressions/misc/hll_sketch_estimate.sql (Spark 4.0+, Parquet dictionary on and off). Sketches are built by Spark for lgK 4/12/21 and 5 to 10000 distinct values, plus empty, NULL, and hll_union_agg results. The estimate query must run natively and match Spark. An invalid-buffer case is query ignore(...) and linked to [Bug] hll_* errors surface as Comet errors instead of HLL_INVALID_INPUT_SKETCH_BUFFER / HLL_UNION_DIFFERENT_LG_K #7549.
  • Dictionary encoding: the native Parquet scan decodes dictionary columns to the required Binary type before any expression sees them (pinned by dictionary_columns_are_unwrapped_to_required_types in native/core/src/parquet/schema_adapter.rs, the same guarantee feat: run length on binary input natively #5874 relied on for length), so spark_hll_sketch_estimate always receives a BinaryArray. The parquet.enable.dictionary=true run of this test checks that end to end.
• CometExpressionSuite: new test checks that hll_sketch_estimate runs natively with no config and that hll_sketch_agg falls back by default with the compact-layout reason. Existing HLL tests no longer set HllSketchEstimate.allowIncompatible.

Local checks (no Maven build locally, CI runs the suites):

• scalafmt --config scalafmt.conf --test (v3.6.1) on the changed Scala files: passed.
• npx prettier --check on the changed Markdown files: passed.

Requesting the run-spark-4.0-tests / run-all-spark-profiles labels, since the serdes only apply on Spark 4.x.