Destructure proto hooks for the Arrow and Avro sources #24622

Scope

4 hooks (2 encoders, 2 decoders).

Notes

Both nodes carry only base_conf on the wire. Every remaining field (table_schema, projection, batch_size, metrics) is either carried by the base config, set at execution time, or runtime state -- so this group should be a clean, purely mechanical conversion. It is a good first issue for the EPIC.

Why

Serde hooks that read state through getters or self.field make an added field invisible to serialization: nothing breaks at compile time, the field simply stops round-tripping, and Debug-comparing round-trip tests do not notice. HashJoinExec::fetch was lost exactly this way (#24165), and #24609 is a second live instance found by applying the convention to one file.

#24164 established the fix -- exhaustive destructuring in both directions -- and applied it to the join plans. The physical-plan plan nodes are done. physical-expr and the datasource* crates were never converted.

What to do

For each hook in scope:

1. In try_to_proto, start with an exhaustive let Self {{ .. }} -- no .. rest pattern. Fields that are genuinely not serialized bind to _ with a short comment saying why (derived at construction, runtime state, recomputed on decode, carried by a parent message).
2. In try_from_proto, destructure the prost-generated node struct the same way, so adding a field to datafusion.proto is a compile error in every decoder.
3. If the destructure turns up a field that should round-trip but has no wire representation, add it to the message and cover it with a test that fails without the fix.

Definition of done

• Every field of the plan/expression struct is either serialized or bound to _ with a reason.

• Every field of the prost node is destructured in the decoder.

• Any newly serialized field has a round-trip test, verified to fail before the fix.

• Part of EPIC: destructure proto serde hooks in the datasource crates #24612.