fix: Set Substrait aggregation phase to INITIAL_TO_RESULT

[namanjain24-sudo]

Which issue does this PR close?

• Closes Substrait producer emits AGGREGATION_PHASE_UNSPECIFIED for every aggregate and window function #25100.

Rationale for this change

The Substrait producer never sets phase on the aggregate and window function
calls it emits, so every call carries AGGREGATION_PHASE_UNSPECIFIED.

That is not the same as leaving the field out. The spec gives the value a
meaning, and it is not the one these plans need:

enum AggregationPhase {
  // Implies `INTERMEDIATE_TO_RESULT`.
  AGGREGATION_PHASE_UNSPECIFIED = 0;
  ...
  // A complete invocation: the function should aggregate the given set of
  // inputs to yield a single return value. This style must be used for
  // aggregate or window functions that are not decomposable.
  AGGREGATION_PHASE_INITIAL_TO_RESULT = 3;

Both AggregateFunction.phase and Expression.WindowFunction.phase are
documented as Required. Must be set to INITIAL_TO_RESULT for ... that are not decomposable.

A LogicalPlan::Aggregate is always a complete aggregation over its input
rows. The partial/final split is a physical planning concern, and the logical
producer has no notion of it, so INITIAL_TO_RESULT is the phase these plans
should declare. What they declare instead carries the spec meaning
INTERMEDIATE_TO_RESULT: that the arguments are already intermediate state to
be combined.

This stays invisible to a DataFusion-to-DataFusion round trip because the
consumer never reads the field (#24967). A consumer that does honour the
declaration reads a complete aggregation as one whose arguments are already
partial state.

What changes are included in this PR?

Set phase to AGGREGATION_PHASE_INITIAL_TO_RESULT at the two producer call
sites that emit it:

• from_aggregate_function in producer/expr/aggregate_function.rs
• make_substrait_window_function in producer/expr/window_function.rs

No other producer site emits the field, and the consumer does not read it, so
nothing else changes.

What is the testing strategy for this PR?

New test aggregate_and_window_functions_declare_initial_to_result in
datafusion/substrait/tests/cases/serialize.rs. It produces plans for a bare
aggregate, a grouped aggregate, and a window function, then walks the produced
protobuf directly and asserts the phase on every aggregate and window function
call, rather than round-tripping through the consumer (which ignores the
field, so a round trip could not catch this).

Verified the test fails without the producer change:

assertion `left == right` failed: phase for `SELECT sum(a) FROM data`
  left: 0
 right: 3

Are there any user-facing changes?

Plans produced by to_substrait_plan now declare
AGGREGATION_PHASE_INITIAL_TO_RESULT instead of
AGGREGATION_PHASE_UNSPECIFIED on aggregate and window function calls. This
is a fix to the emitted protobuf rather than a Rust API change. Consumers that
read phase will now see the correct declaration; consumers that ignore it,
including DataFusion's own, are unaffected.

[Xuanwo]

Nice fix!

[namanjain24-sudo]

Nice fix!

Thank you, @Xuanwo