Fix DISTINCT on ENUM/SET columns by making enums/set hashable

[codex-maintainers]

Description

Vitess’ DISTINCT (and grouping) primitives rely on evalengine.NullsafeHashcode
to deduplicate rows coming back from scatter queries. While most value types
implement the hashable interface so our default hashing can recognize them,
both evalEnum and evalSet did not. The result is that any query that tries
to sort or de-dup on an ENUM or SET column across shards would error out
with unexpected type ENUM.

This PR adds Hash implementations for both enum and set evals so that
DISTINCT, GROUP BY, hash join and similar code paths can hash these types
properly. For enum/set values we hash the numeric ordinal/bitset when all
values are known, and fall back to hashing the raw string under a binary
collation when we can't resolve the value.

To catch this in end-to-end tests, the aggregation test keyspace is updated
with a simple example table that has an enum column, and a new assertion is
added to the DISTINCT test to exercise SELECT DISTINCT foo FROM example;,
which fails prior to the hashable fix and passes afterwards.

Related Issue(s)

Fixes #17676

Checklist

• "Backport to:" labels have been added if this change should be back‑ported to release branches
• If this change is to be back‑ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

None.

---

This PR was generated by an AI system in collaboration with maintainers @rbranson, @GuptaManan100, @rohit-nayak-ps, @systay.

[vitess-bot]

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[dbussink]

@gopoto Looks like you accidentally committed a bunch of test files here that shouldn't be?

[codex-maintainers]

Removed the vtroot_6701 tmp and topo directories that were committed in error; these logs and wal files were artifacts of a previous test run and shouldn't be in the tree.

[arthurschreiber]

I guess this will have to be backported to oder versions as well? 🤔

[arthurschreiber]

Is there a way to test the "unknown set" and "unknown enum" value cases?

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Project coverage is 67.54%. Comparing base (526665e) to head (a98ee42).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/evalengine/eval_enum.go	77.77%	2 Missing ⚠️
go/vt/vtgate/evalengine/eval_set.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17936      +/-   ##
==========================================
- Coverage   67.56%   67.54%   -0.02%     
==========================================
  Files        1597     1597              
  Lines      259780   259851      +71     
==========================================
+ Hits       175516   175529      +13     
- Misses      84264    84322      +58     

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[dbussink]

@gopoto Is this ready for review?