EPIC: destructure proto serde hooks in physical-expr #24611

Background

#24164 established a convention for the per-type proto hooks introduced by EPIC #23494: try_to_proto starts with an exhaustive let Self { .. } (no ..), and try_from_proto destructures the prost-generated node the same way. Adding a field to the struct then becomes a compile error in the encoder, and adding a field to datafusion.proto becomes a compile error in every decoder.

That PR applied the convention to the join plans, and the rest of the physical-plan plan nodes follow it today. physical-expr was never converted: every expression hook still reads self.field or a getter.

Why this matters

A hook that reads state field-by-field makes 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 see it. This is not hypothetical:

• HashJoinExec::fetch was dropped this way (fixed in fix(proto): preserve HashJoinExec fetch across serialization #24165).
• CsvSource terminator is a live instance found by applying the convention to a single file (CsvSource::with_terminator is silently dropped when a physical plan is serialized #24609).
• Checking each un-destructured encoder in this crate against its struct's fields turned up three more, listed in the sub-issues below.

Inventory

42 un-destructured hooks in this EPIC's scope (21 encoders, 21 decoders) across 21 files.

Sub-issues

• Destructure proto hooks for leaf and unary physical expressions #24613 Destructure proto hooks for leaf and unary physical expressions
• Destructure proto hooks for binary and pattern physical expressions #24614 Destructure proto hooks for binary and pattern physical expressions
• Destructure proto hooks for cast physical expressions #24615 Destructure proto hooks for cast physical expressions
• Destructure proto hooks for CASE and IN-list physical expressions #24616 Destructure proto hooks for CASE and IN-list physical expressions -- refactor(proto): destructure CASE and IN-list expression serde hooks #24700 (merged)
• Destructure proto hooks for lambda physical expressions #24617 Destructure proto hooks for lambda physical expressions -- refactor(proto): destructure lambda expression serde hooks #24775 (merged)
• Destructure proto hooks for dynamic filter and scalar subquery expressions #24618 Destructure proto hooks for dynamic filter and scalar subquery expressions
• Destructure proto hooks for sort expressions and partitioning #24619 Destructure proto hooks for sort expressions and partitioning

Field drops already confirmed

BinaryExpr::fail_on_overflow is the one worth fixing first, since it is the only one that changes results rather than metadata or performance.

Public API to consider deprecating alongside this work

None found in physical-expr. The getters these hooks call (CastExpr::cast_type, CastExpr::expr, LambdaExpr::body, LambdaExpr::params, LambdaVariable::index, ...) all have other callers in the workspace, so converting the hooks to destructuring does not orphan any of them.

For contrast, the equivalent review of the already-converted plan nodes did find dead accessors -- see the corresponding section on the datasource EPIC for the method and results.