fix(bootstrapper): preserve BUILD_SDIST requirement type for sdist build deps

Split the combined build_backend_deps | build_sdist_deps call into
separate _create_dep_work_items calls with their correct RequirementType
so BUILD_SDIST edges are not mislabeled as BUILD_BACKEND.

Co-Authored-By: Claude <claude@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Rohan Devasthale <rdevasth@redhat.com>

Author: rd4398

src/fromager/bootstrapper.py

@@ -1486,12 +1486,19 @@ def _phase_prepare_build(self, item: WorkItem) -> list[WorkItem]:
             build_env=item.build_env,
         )
 
-        dep_items = self._create_dep_work_items(
-            item.build_backend_deps | item.build_sdist_deps,
+        backend_items = self._create_dep_work_items(
+            item.build_backend_deps,
             RequirementType.BUILD_BACKEND,
             item.req,
             item.resolved_version,
         )
+        sdist_items = self._create_dep_work_items(
+            item.build_sdist_deps,
+            RequirementType.BUILD_SDIST,
+            item.req,
+            item.resolved_version,
+        )
+        dep_items = backend_items + sdist_items
 
         item.phase = BootstrapPhase.BUILD
         return dep_items + [item]
@@ -1673,6 +1680,12 @@ def _handle_phase_error(
                 f"failed to bootstrap: {type(err).__name__}: {err}"
             )
             self.ctx.dependency_graph.remove_dependency(pkg_name, item.resolved_version)
+            self._seen_requirements.discard(
+                self._resolved_key(item.req, item.resolved_version, "sdist")
+            )
+            self._seen_requirements.discard(
+                self._resolved_key(item.req, item.resolved_version, "wheel")
+            )
             self.ctx.write_to_graph_to_file()
             return []