fix: prevent panic in uniqueItems validation for unhashable types (#1042)

[ArdyJunata]

Summary

Fixes #1042

Problem

The uniqueItems check in handleArray used map[any]struct{} as a hash set. When array items contain non-hashable types such as map[string]interface{} — produced by malformed JSON like [{}] for a []string field — inserting into the map panics:

http: panic serving [::1]:59220: hash of unhashable type: map[string]interface {}

This caused the server to return no response at all instead of a 422 Unprocessable Entity. The panic kills the goroutine serving the request.

Reproduction:

curl -X POST -H 'Content-Type: application/json' \
  -d '{"arrayOfStrings": [{}]}' \
  http://localhost:8888/demo
# Server logs: panic serving ...: hash of unhashable type: map[string]interface {}

Fix

Introduce a uniqueItemsContains helper that:

1. Fast path: attempts an O(1) map[any] lookup (works for all hashable types — strings, ints, bools, etc.)
2. Slow path: a deferred recover() catches the panic for unhashable types and falls back to an O(n) reflect.DeepEqual linear scan

This means:

• ✅ Normal arrays of hashable items → same O(n) performance as before
• ✅ Arrays with maps/slices → graceful O(n²) fallback, no panic
• ✅ Duplicate unhashable objects → still detected correctly
• ✅ Malformed [{}] for []string → proper 422 expected string error

Tests

Three regression tests added to validate_test.go:

Test	Verifies
uniqueItems with unhashable object element must not panic	[{}] for []string returns 422, not a panic
uniqueItems detects duplicate unhashable objects	[{"a":1},{"a":1}] is still detected as non-unique
uniqueItems with mixed hashable and unhashable elements	["ok", {"k":"v"}] does not panic

All existing tests continue to pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (2b69054) to head (a093467).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1045      +/-   ##
==========================================
+ Coverage   93.14%   93.16%   +0.01%     
==========================================
  Files          23       23              
  Lines        4901     4913      +12     
==========================================
+ Hits         4565     4577      +12     
  Misses        272      272              
  Partials       64       64              

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