[AURON #2176] Implement native support for lead window function

[weimingdiit]

Which issue does this PR close?

Closes #2176

Rationale for this change

Auron’s native window support previously covered rank-like functions and a subset of aggregate window functions, but did not support offset-based window functions such as lead(...).

This PR extends native window coverage with a conservative first step:

• support lead(...)
• preserve Spark-compatible behavior for input, offset, and default
• keep unsupported semantics out of the native path rather than approximating them incorrectly

What changes are included in this PR?

This PR:

• adds Lead handling in NativeWindowBase
• extends the protobuf/planner window function enum with LEAD
• adds native planner support to decode LEAD into the native window plan
• introduces a native LeadProcessor in datafusion-ext-plans
• evaluates lead using Spark-compatible offset/default/null behavior
• adds a full-partition processing path for lead so that lookahead works correctly across input batches
• adds Rust regression coverage for cross-batch lead
• adds Scala regression tests for:
  • native lead(...) execution
  • Spark fallback for lead(... ) IGNORE NULLS

The native implementation supports Spark semantics for:

• lead(input)

  • default offset is 1
  • default value is null

• lead(input, offset, default)

  • returns the value of input at the offsetth row after the current row in the same window partition
  • if the target row exists and input there is null, returns null
  • if the target row does not exist, returns default

Supported scope in this PR:

• standard RESPECT NULLS behavior

Not supported natively in this PR:

• IGNORE NULLS

Unsupported IGNORE NULLS queries continue to fall back to Spark to preserve correctness.

Are there any user-facing changes?

Yes.
Queries using lead(...) can now remain on Auron’s native window execution path when they use supported semantics.
Queries using unsupported lead(... ) IGNORE NULLS behavior will continue to fall back to Spark.

How was this patch tested?

CI.