Add ImHashMap defect repro tests

[dzmitry-lahoda]

Summary

Add focused repro tests for concrete ImHashMap and PartitionedHashMap defects found during audit.

Coverage added

• custom partition counts smaller than 16 fail with default mask-based indexing
• custom partition counts larger than 16 silently use only the first 16 partitions by default
• concurrent PartitionedHashMap.AddOrKeep can overwrite on CAS retry
• BuildFromDifferent rejects same-hash distinct keys despite the current API contract mentioning only key uniqueness
• BuildUnchecked(IEnumerable<HKV<...>>) does not dispose the source enumerator
• BuildUnchecked<int, V> produces entries incompatible with int-specialized accessors

Validation

• dotnet test test/ImTools.UnitTests/ImTools.UnitTests.csproj -p:CILinux=true -f net11.0 --filter "Custom_partition_count_smaller_than_16_breaks_default_operations|Custom_partition_count_larger_than_16_still_uses_only_first_16_partitions_by_default|Concurrent_AddOrKeep_may_overwrite_existing_value_after_compare_exchange_retry|BuildFromDifferent_rejects_distinct_keys_with_the_same_hash_even_though_the_api_mentions_only_key_uniqueness|BuildUnchecked_does_not_dispose_source_enumerator|BuildUnchecked_with_int_keys_creates_entries_incompatible_with_int_specialized_accessors"
• dotnet test test/ImTools.UnitTests/ImTools.UnitTests.csproj -p:CILinux=true -f net11.0 --filter "ImHashMapTests|PartitionedHashMapTests"

[dzmitry-lahoda]

AI generated, seems at least 2 legit

[dadhi]

I see :) I will check