fix: support arbitrary types in task dependencies

Enable tasks to accept ANY type implementing Clone + Send + Sync as
dependencies, without requiring users to implement ExtractInput trait.

The #[task] macro now generates task-specific extraction logic inline
via a new extract_and_run() method. This leverages the macro's perfect
compile-time knowledge of parameter types to handle extraction for:
- Zero parameters (unit type, source tasks)
- Single parameter (any Clone + Send + Sync type)
- Multiple parameters (tuples, awaited concurrently)

Previously, only hardcoded primitive types and specific stdlib types
worked. Custom user types required manual ExtractInput implementations.
Now custom types work automatically with zero API changes.

Changes:
- Add extract_and_run() method to Task trait (internal/hidden)
- Generate inline extraction in #[task] macro based on parameter count
- Remove ExtractInput bound from DagRunner::add_task()
- Retain ExtractInput for internal task_fn() test helper
- Add comprehensive custom type tests

No breaking changes - purely internal implementation improvement.

src/extract.rs

@@ -28,6 +28,11 @@ use futures::channel::oneshot;
 ///
 /// Type erasure occurs only at the ExecutableNode trait boundary - by the time
 /// we're in ExtractInput, we know the concrete type and can safely downcast.
+///
+/// NOTE: This trait is now primarily used by the internal `task_fn` test helper.
+/// The `#[task]` macro generates inline extraction logic via `extract_and_run()`,
+/// which allows ANY type to work without requiring ExtractInput implementations.
+#[allow(dead_code)]
 pub(crate) trait ExtractInput: Sized + Clone {
     fn extract_from_channels(
         receivers: Vec<Box<dyn Any + Send>>,