Missing `with_key_clause` definition in InfluxQL spec

The spec already defines `with_tag_clause` with exactly that `WITH KEY (...)` syntax (line 1274 in the enterprise spec). The references I changed (`show_tag_keys_stmt`, `show_tag_values_cardinality_stmt`, `show_tag_values_exact_cardinality_stmt`) all pointed to `with_key_clause`, which was never defined — so I updated them to use the defined `with_tag_clause` production. If a separate `with_key_clause` production should be added to the grammar, let me know and I can add it.

See additional comments in https://github.com/influxdata/docs-v2/pull/6932#discussion_r2931300307_

Start with the enterprise spec at line 1274 and inspect the show_tag_keys_stmt, show_tag_values_cardinality_stmt, and show_tag_values_exact_cardinality_stmt references. Confirm that they use the defined with_tag_clause production, or determine whether a separate with_key_clause production is required; done means the referenced grammar production is defined and consistent.

Written by the indexing model from the issue text.