fix(proto): preserve empty projection when ser/de MemoryScanExec

[Dandandan]

Which issue does this PR close?

Finishes the work started in #23082 (HashJoinExec, NestedLoopJoinExec) and #21885 (FilterExec) — MemoryScanExec was the last physical plan node left with an ambiguous projection.

Rationale for this change

MemorySourceConfig's projection is Option<Vec<usize>> but is encoded as a bare repeated uint32:

// encode
let proto_projection = source_conf.projection().as_ref().map_or_else(Vec::new, |v| ...);
// decode
let projection = if !scan.projection.is_empty() { Some(...) } else { None };

So Some(vec![]) — project away every column — serializes exactly like None and always decodes back as None, and the decoded scan reports every column again.

What changes are included in this PR?

Encode an empty projection with the single-element [u32::MAX] sentinel — the same convention HashJoinExec and NestedLoopJoinExec already use — and decode it back to Some(vec![]). Every other state is unchanged on the wire, so None and non-empty projections round-trip exactly as before.

Are these changes tested?

Yes

Are there any user-facing changes?

No API change. The wire format changes only for the previously-unrepresentable Some(vec![]) case, matching what the joins already do.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.89%. Comparing base (db0c31b) to head (d298fba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24084      +/-   ##
==========================================
- Coverage   80.89%   80.89%   -0.01%     
==========================================
  Files        1102     1102              
  Lines      376111   376107       -4     
  Branches   376111   376107       -4     
==========================================
- Hits       304251   304246       -5     
+ Misses      53753    53750       -3     
- Partials    18107    18111       +4     

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