test: skip count distinct spill memory test under forced hash collisions

[rluvaton]

Which issue does this PR close?

Follow-up to #24888, which broke the cargo test hash collisions CI job on main.

Rationale for this change

The memory limit test added in #24888 fails when built with force_hash_collisions. Every key hashes to the same value there, so the hash repartition sends all 64 groups to one final stage. That single table needs 5.3 MB against the test's 4 MB pool, and it has nothing reserved yet, so there is nothing to spill. It fails no matter how well the accumulator releases memory, which is what the test is actually about.

I tried a few ways to keep it running under the feature first:

• Bigger limit for the collision build. Below 5.3 MB it dies on that one state batch; at 6 MB and up nothing spills, so the unfixed accumulator passes too and the test asserts nothing. Nothing in between.
• Single partition, no repartition at all. Same wall. With 64 groups the whole distinct state lives in 64 rows, so total state and one batch are the same 5.3 MB. Also 82s instead of 0.18s.
• More groups, to spread the state over more batches. With every key in one hash bucket, interning goes quadratic: 4096 groups did not finish in 400s.
• More rows (800k), to make total state exceed one batch. Fails even with the fix.

They all hit the same thing: under forced collisions the total state and a single batch are the same size, and the pool would have to sit above one and below the other.

What changes are included in this PR?

The test and its helpers move into a module gated on not(feature = "force_hash_collisions").

What is the testing strategy for this PR?

cargo test -p datafusion --features force_hash_collisions --test core_integration count_distinct_releases runs 0 tests. Without the feature it still runs and passes.

Are there any user-facing changes?

No.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.63%. Comparing base (a6f1dce) to head (e484973).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24918   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files        1123     1123           
  Lines      409963   409963           
  Branches   409963   409963           
=======================================
+ Hits       334685   334687    +2     
+ Misses      55605    55598    -7     
- Partials    19673    19678    +5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Weijun-H]

LGTM

Non-blocking wording nit: filter_pushdown.rs adapts expectations under the feature rather than skipping the test, so consider removing that comparison from the PR description.