[rust-compiler] Fix CompilerDiagnostic::todo() to produce ErrorDetail variant

Removed the flat-loc serialization hack from log_error, compiler_error_to_info,
and log_errors_as_events. Instead fixed the root cause: the From<CompilerDiagnostic>
for CompilerError impl now converts Todo-category diagnostics into
CompilerErrorOrDiagnostic::ErrorDetail (matching TS's CompilerError.throwTodo()
which creates CompilerErrorDetail with loc directly). Invariant-category diagnostics
remain as CompilerErrorOrDiagnostic::Diagnostic with sub-details. 1723/1723 passing.