fix(ffi): don't discard downcast-delegating wrappers of foreign plans

[timsaucer]

Which issue does this PR close?

• Closes FFI_ExecutionPlan::new silently discards transparent wrapper nodes #25717.

Rationale for this change

A transparent wrapper ExecutionPlan (one that implements downcast_delegate, such as datafusion-tracing's InstrumentedExec) around a ForeignExecutionPlan is silently dropped when it is passed back across the FFI boundary. For example, installing the tracing instrumentation rule via FFI leaves the plan un-instrumented. Also, when such a wrapper is the child of a foreign plan, it does not receive the runtime handle.

What changes are included in this PR?

• FFI_ExecutionPlan::new: the "already foreign" shortcut now checks the concrete type via Any instead of the delegate-aware downcast_ref, so it only fires for a real ForeignExecutionPlan.
• pass_runtime_to_children: the child check also uses Any, so a local wrapper child of a foreign parent is re-wrapped with the runtime.
• plan_is_foreign intentionally stays delegate-aware, since a delegating wrapper typically forwards replace_children to the foreign plan and its children still need the runtime. A comment explains this.

What is the testing strategy for this PR?

• Added an optional downcast_delegate mode to the existing EmptyExec test plan (with_downcast_delegate).
• Added test_ffi_execution_plan_delegating_wrapper, which checks that the wrapper survives FFI_ExecutionPlan::new and that a wrapper child of a foreign parent gets the runtime. Reverting either fix on its own makes the test fail.

Are there any user-facing changes?

No API changes. Wrapper plans that implement downcast_delegate are now kept when they cross the FFI boundary instead of being discarded.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.55%. Comparing base (991fd23) to head (cf21434).

Files with missing lines	Patch %	Lines
datafusion/ffi/src/execution_plan.rs	89.74%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25749      +/-   ##
==========================================
- Coverage   82.55%   82.55%   -0.01%     
==========================================
  Files        1141     1141              
  Lines      440209   440246      +37     
  Branches   440209   440246      +37     
==========================================
+ Hits       363409   363426      +17     
- Misses      54829    54842      +13     
- Partials    21971    21978       +7     

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[namanjain24-sudo]

FYI, closed #25645 in favor of this one — it covers the same fix plus pass_runtime_to_children.

[alamb]

I merged up from main to resolve a conflict