chore(django-google-spanner): unconditionally quote all identifiers in quote_name #19072

Follow-up to #19069.

Once prerequisite schema.py fixes are in place, DatabaseOperations.quote_name() should unconditionally quote all identifiers instead of leaving unreserved regular ASCII identifiers unquoted (so reserved-keyword tracking via SPANNER_RESERVED_KEYWORDS is no longer needed).

Tasks

• Fix OpenTelemetry model_name span attributes in django_spanner/schema.py to pass unquoted model._meta.db_table instead of calling self.quote_name(...).
• Ensure check-constraint DDL quoting (fix(django-google-spanner): fix column check constraint DDL #19070) is merged.
• Update unit tests expecting unquoted unreserved identifiers.
• Remove the _REGULAR_IDENTIFIER_RE / SPANNER_RESERVED_KEYWORDS fast-path in DatabaseOperations.quote_name().