[planner fix 16.0] make unknown column an error only for sharded queries

[systay]

Description

Backport of #12704

In earlier releases, the vtgate planner would be lenient with unknown columns, letting the underlying MySQL if there was a real problem.

This PR re-introduces this behaviour. On queries that only touch a single unsharded keyspace, the gen4 planner will allow missing column information.

Related Issue(s)

Fixes #12548

[vitess-bot]

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