fix: Compose tasks when composing turbo.json (#11248)
### What
This PR fixes task discovery and resolution when Package Configurations
use the `extends` field to inherit from non-root packages.
### Why
Previously, when a Package Configuration extended another non-root
package (e.g., `"extends": ["//", "shared-config"]`), tasks defined in
the extended package were not properly discovered or resolved.
### How
The fix modifies the engine builder to properly traverse the extends
chain when:
1. Collecting tasks (`collect_tasks_from_extends_chain`): A new
recursive function traverses the extends chain from any workspace,
collecting all task names from each extended package. This ensures
add_all_tasks discovers inherited tasks.
2. Checking task definitions (`has_task_definition_in_run_inner`):
Updated to recursively check the extends chain for task definitions, not
just the immediate workspace and root.
### Testing Instructions
I wrote tests covering these situations but you're encouraged to test
yourself if you'd like:
- Basic extends inheritance
- Deep extends chains (A→B→C→D→root)
- Diamond inheritance patterns
- Cyclic extends handling
- Missing turbo.json fallback behavior
