…tOne

The old code looks like a copy-paste mistake.

Address the FIXMEs in crates/hir-ty/src/lower.rs where
next_ty_var, next_const_var, and next_region_var silently
returned error types without emitting diagnostics when
inference variables are not allowed (e.g., `_` in type aliases,
consts, statics, struct fields).

- Add InferVarsNotAllowed variant to TyLoweringDiagnostic enum
- Call push_diagnostic in the three next_*_var functions for any
  non-dummy span
- Convert TyLoweringDiagnostic from struct+kind to flat enum with
  per-variant source (TypeRefId for PathDiagnostic, Span for
  InferVarsNotAllowed)
- Add span_syntax helper resolving all Span variants to AST nodes
- Extract expr_syntax/pat_syntax/type_syntax/span_syntax closures
  from inference_diagnostic into associated functions on AnyDiagnostic
- Add InferVarsNotAllowed HIR diagnostic struct with
  InFile<SyntaxNodePtr> node
- Add ide-diagnostics handler with E0121 error code and tests

fix: RunnableKind::Test should map to project_json::RunnableKind::TestOne

`pat()` does not parse or patterns.

feat(diagnostics): emit error for infer vars in non-inference contexts

fix: Parse OR pattern types

internal: Align MIR ProjectionElem more with rustc's version and simplify it