Break the exir backend_api <-> program import cycle (#24802)

### Summary
**Stacked on #24799.** Until that lands this branch also contains its
commits; review only the last commit, 121f6d6.

`exir/backend/backend_api.py` imports
`executorch.exir.program._fake_program`, which runs
`exir/program/__init__.py` and so `_program.py`. `_program.py` imported
`MethodProgramsPartitionerSpec` and `to_backend` from the still
half-initialised `backend_api` at module scope:

```
ImportError: cannot import name 'MethodProgramsPartitionerSpec' from partially initialized module 'executorch.exir.backend.backend_api' (most likely due to a circular import)
```

Normally `exir/__init__.py` loads `program` first, so this only fails
when `backend_api` is the first `executorch.exir` import, for example
under lazy imports (Cinder lazy imports on Python 3.14) when a backend
imports `to_backend` directly. On main, that import order fails earlier
on the `capture` cycle fixed in #24799, which is why this is stacked.

`_program.py` uses those two names only in
`EdgeProgramManager.to_backend`, so the import moves into that method.
Nothing imports them through `_program`.

This PR was authored with AI assistance (Claude Code).

### Test plan
Added `executorch.exir.backend.backend_api` to
`exir/tests/test_import_order.py` (added in #24799). It fails with the
error above without the `_program.py` change and passes with it. The
other entry points in that test still pass.

`python -m unittest executorch.exir.program.test.test_program
executorch.exir.backend.test.test_backends_lifted
executorch.exir.backend.test.test_partitioner
executorch.backends.xnnpack.test.ops.test_add` gives the same results
with and without this commit, apart from the new import-order case. The
shared failures come from my local runtime build, which lacks
`BackendWithCompilerDemo` and some kernels such as
`aten::_embedding_bag.out`.

Author: JakeStevens

exir/program/_program.py

@@ -23,10 +23,6 @@
 from executorch.exir._serialize._serialize import serialize_for_executorch
 from executorch.exir._serialize.data_serializer import DataSerializer
 from executorch.exir._warnings import experimental
-from executorch.exir.backend.backend_api import (
-    MethodProgramsPartitionerSpec,
-    to_backend,
-)
 from executorch.exir.backend.partitioner import Partitioner
 from executorch.exir.capture._config import EdgeCompileConfig, ExecutorchBackendConfig
 from executorch.exir.delegate import executorch_call_delegate, is_lowered_module
@@ -1725,6 +1721,13 @@ def to_backend(
             EdgeProgramManager: A copy of the calling EdgeProgramManager with the
             specified subgraphs lowered.
         """
+        # backend_api imports executorch.exir.program (via program._fake_program),
+        # so importing it at module scope is circular.
+        from executorch.exir.backend.backend_api import (
+            MethodProgramsPartitionerSpec,
+            to_backend,
+        )
+
         new_edge_programs: Dict[str, ExportedProgram] = {}
         method_to_partitioner: Dict[str, Partitioner] = {}
         if not isinstance(partitioner, dict):

exir/tests/test_import_order.py

@@ -28,6 +28,7 @@ def test_import_without_exir_init(self) -> None:
             "executorch.exir.verification.verifier",
             "executorch.exir.program",
             "executorch.exir._serialize._serialize",
+            "executorch.exir.backend.backend_api",
         ):
             with self.subTest(module=module):
                 completed = subprocess.run(