fix(types): normalize nullable any return bindings

[dd-david-levin]

Summary

Normalize a nullable anyN? occurrence to the required base binding when deriving polymorphic return types, then let the return occurrence reapply its declared nullability.

This implements the return-substitution side of the semantics documented in substrait-io/substrait#960. In particular:

list<string?> against list<any1?> binds any1 = string
list<i64?> against list<any2?> binds any2 = i64
map<any1, any2?> resolves to map<string, i64?>

Without this change, the map key incorrectly remains nullable and resolves as map<string?, i64?>.

Details

• Normalize nullable anyN? source occurrences in unwrapAnyTypeWithName.
• Preserve the return occurrence's declaration, so anyN remains required and anyN? is nullable.
• Add direct required/nullable substitution tests and a composite map regression test.
• Make IntervalDayType.WithNullability honor the Type interface's copy contract. This prevents return derivation from mutating an input argument while normalizing its binding.

This is complementary to draft #244. That PR addresses broader argument matching and constrained-binding consistency; this PR is narrowly about extracting the correct binding for return-type substitution.

Testing

go test ./...
git diff --check

Both pass.

---

This PR was developed with AI assistance. All changes have been reviewed, and I take full responsibility for this contribution.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.45%. Comparing base (985d911) to head (bfc7b50).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
+ Coverage   70.41%   70.45%   +0.03%     
==========================================
  Files          48       48              
  Lines       11087    11090       +3     
==========================================
+ Hits         7807     7813       +6     
+ Misses       2910     2908       -2     
+ Partials      370      369       -1     

☔ 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.