fix: delegate the remaining codec methods in ComposedPhysicalExtensionCodec

[sainad2222]

Which issue does this PR close?

• Closes ComposedPhysicalExtensionCodec implements only 6 of the 12 PhysicalExtensionCodec methods #24829.

Rationale for this change

See #24829.

What changes are included in this PR?

Delegates the six missing methods to the child codecs.

One wrinkle: the four by-name hooks (udf, udaf, udwf,
higher_order_function) default to Ok(()) meaning "no payload, encode by
name", and decode only consults the registry when no payload is present. Naive
delegation wraps an empty blob and strands those functions, invisible at
position 0, where the tuple prost-encodes to zero bytes. Those four now use a
new encode_protobuf_by_name_aware, which emits a payload only when a codec
actually wrote bytes. try_encode_expr keeps the plain path; its default is an
error rather than Ok(()).

What is the testing strategy for this PR?

Four tests in physical_plan::tests::function_serde; two of them fail against
naive delegation.

Are there any user-facing changes?

try_encode_udf / try_encode_udaf were already delegated but carried the same
empty-payload bug, so they move to the new helper too. Happy to split that out.
No public API signatures changed.

[ryux1]

The by-name helper handles empty-success delegation carefully. The newly added expression path still inherits a separate composition bug.

[ryux1]

The new buffer reset isolates each codec attempt, and the regression covers a codec that writes before rejecting followed by one that succeeds. This addresses my concern. Thanks!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.63184% with 106 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.50%. Comparing base (587e74c) to head (f9aedea).
⚠️ Report is 143 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	73.63%	80 Missing and 26 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24973      +/-   ##
==========================================
+ Coverage   81.97%   82.50%   +0.53%     
==========================================
  Files        1137     1140       +3     
  Lines      429922   438917    +8995     
  Branches   429922   438917    +8995     
==========================================
+ Hits       352419   362132    +9713     
+ Misses      56453    54882    -1571     
- Partials    21050    21903     +853     

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[timsaucer]

I think the core change makes a lot of sense, but I am worried about code drift. I think we can tighten it up a bit. My agent also had a few comments.

🤖 note:

Compatibility. Worth a line in the PR description: for a composed codec where the accepting codec sat at position > 0 and wrote no bytes, the emitted bytes change (tuple → nothing). Old bytes still decode fine under the new code, and those cases were undecodable before an

[timsaucer]

I think we should fix up the decode by name issue in the previous thread while we're working on this. It's looking good!