Avoid reparsing attribute syntax to resolve canonical names

[jonathanvdc]

The Binder currently resolves attribute definition names by converting the syntax node back to text and reparsing it:

TryGetAttributeDefinitionName(syntaxNode.ToString())

That is brittle and unnecessary. This path only applies to custom attributes, so we should extract the name directly from the syntax node instead of round-tripping through text.

File: src/MLIR/Semantics/Binder.cs
Context: the attribute binding path near the canonicalName lookup.

Suggested follow-up:

• inspect the custom attribute syntax node shape directly
• derive the canonical name without ToString()
• keep the lookup logic aligned with the actual parsed syntax tree