[VL] Preserve ordinals for nested Substrait field references

[malinjawi]

What changes are proposed in this pull request?

Substrait identifies nested struct fields by ordinal, but the Velox converter
currently represents every level as a name-based field access. Struct fields
may have duplicate or empty names, so name lookup can select the wrong child.

Add a dedicated conversion path for full Expression::selection expressions.
The root input remains a FieldAccessTypedExpr, while nested struct fields use
DereferenceTypedExpr with their Substrait ordinals. Existing direct
FieldReference callers remain unchanged, and invalid references continue to
fail with a user error.

Nested aggregate masks are rejected explicitly because Velox aggregation masks
must reference a top-level input field. This prevents an unsupported nested mask
from being silently dropped.

Simplified evaluation of the resulting ordinal dereference also requires
Velox #18537.

How was this patch tested?

• Added regression coverage for duplicate unnamed nested fields.
• Added validation coverage for invalid ordinals and non-struct traversal.
• Added plan-conversion coverage for top-level and nested aggregate masks.
• Ran ClangFormat 15 and compiled all changed translation units with -Werror.

Was this patch authored or co-authored using generative AI tooling?

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