Better user guidance on failure to capture non-const symbols in OpenQASM

Following up on https://github.com/microsoft/qsharp/pull/2417#discussion_r2093301530, it looks like the error clean up exposed an issue with how `SemanticErrorKind::ExprMustBeConst` is used in `lower_ident_expr`. The error is very helpful, but no longer gets filed because symbol resolution doesn't return actual symbols that are non-const. We need to review the interaction with `try_get_existing_or_insert_err_symbol` to ensure the assumptions are compatible with the `lower_ident_expr`.

Start by reading the lower_ident_expr handling of SemanticErrorKind::ExprMustBeConst and the interaction with try_get_existing_or_insert_err_symbol. Trace how non-const symbols are resolved, then verify that the intended user guidance is emitted when such a symbol is captured in OpenQASM.

Written by the indexing model from the issue text.