Fix virtual fields on deep projection

[cdobre]

Description

When a custom logical schema adds virtual sub-fields to a struct, their names appeared in projection_deep passed to the Parquet reader.  The reader fell back to reading the whole struct (or errored) because the virtual names don't exist in the physical file.

Root cause: scan_plan.parquet_read_schema was derived from the kernel scan which was built with the custom schema, so it also contained virtual sub-fields and could not be used to distinguish physical from virtual. But we can use the table_config.schema to differentiate between virtual and actual physical fields so we can apply a filter which:

• drops any entries which are part of an entirely-virtual top-level structs
• filters sub-field lists to keep physical ones only
• injects the first physical sub-field as a row-count anchor when all requested sub-fields are virtual, so null materialization has the right row count

[github-actions]

ACTION NEEDED

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