Throw ArgumentNullException on StringFnV1AHasher.Hash(null)

[marius-bughiu]

Summary

Closes #71. Replaces the NullReferenceException that StringFnV1AHasher.Hash(null) currently throws with an explicit ArgumentNullException (parameter name "key"), and updates the test that previously pinned the NRE behaviour.

NullReferenceException from a public API is a code-quality wart per .NET framework design guidelines — it conflates a programmer error (unchecked dereference) with what is really a contract violation. The previous test even openly noted the wart: "If you'd prefer an explicit ArgumentNullException, modify the struct." This PR makes that change.

Surface area

The Celerity dictionaries route the out-of-band null / default(TKey) key entry around the hasher and never call Hash(null), so existing dictionary callers see no behaviour change. The new ArgumentNullException only surfaces when:

• callers use StringFnV1AHasher directly (e.g. as a building block in their own collection or a hash-quality test harness), or
• callers plug StringFnV1AHasher into a custom IHashProvider<string> consumer that does not handle the null-key slot itself.

In both cases the new exception is the correct one — and the regression test pins it down.

Files

• src/Celerity/Hashing/StringFnV1AHasher.cs — ArgumentNullException.ThrowIfNull(key) at the top of Hash, plus a full XML doc on the method declaring the exception.
• src/Celerity.Tests/Hashing/StringFnV1AHasherTests.cs — Hash_NullString_ThrowsNullReferenceException is renamed to Hash_NullString_ThrowsArgumentNullException and asserts both the exception type and ParamName == "key".
• CHANGELOG.md — [Unreleased] Fixed entry.

Test plan

• dotnet build clean (0 errors; 55 pre-existing xUnit2013 warnings in unrelated test files)
• CI runs StringFnV1AHasherTests on Linux / Windows / macOS — local runtime is .NET 10 only and the projects target net8.0, so the test execution gate is CI
• Coverage in StringFnV1AHasherTests.Hash_NullString_ThrowsArgumentNullException:
  • Hash(null!) throws ArgumentNullException
  • The thrown exception's ParamName is "key"
• No change to existing dictionary code paths — the out-of-band null/default-key handling already short-circuits the hasher call.

[marius-bughiu]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merged main into the branch and resolved the CHANGELOG.md conflict — the [Unreleased] ### Fixed entry is preserved above the new [1.2.0] section. Build is clean (0 errors). Commit: 004d36a.