Add Unity-aware async correctness diagnostics

[Sebastien Lebreton (sailro)]

Fixes #426

Checklist

• I have read the Contribution Guide ;
• There is an approved issue describing the change when contributing a new analyzer or suppressor ;
• I have added tests that prove my fix is effective or that my feature works ;
• I have added necessary documentation (if appropriate) ;

Short description of what this resolves:

Adds the four async correctness checks highlighted in #426, with support for UniTask and Unity Awaitable as well as .NET Task and ValueTask types.

Changes proposed in this pull request:

• Add UNT0045 and UNT0046 for async-void methods and delegates, using resolved signatures and preserving recognized Unity callback, event-handler, override, and interface contracts.
• Add UNT0047 for task-to-string conversions, including interpolation, concatenation, formatting APIs, and Unity logging.
• Add UNT0048 for null task returns, distinguishing null operations from valid null async results and value-type defaults, including nested functions and expression bodies.
• Reuse TypeSymbolExtensions, ScriptInfo, and existing load-callback detection; preserve existing Unity message matching rather than broadening it to every task-like type.
• Add diagnostic resources, documentation, shared compile-only UniTask test types, and regression coverage.

No automatic code fixes are provided because changing signatures or inserting await can alter caller contracts and execution order. The issue-approval checklist item is left unchecked because the issue does not record explicit approval.

Validation

• Analyzer/test project build succeeded.
• All 347 focused tests passed, covering the new diagnostics, related existing analyzers, and diagnostic ID consistency.
• The full suite, including network-based documentation-link checks, was not run.