fix(bundler): treat an explicit-null manifest field as missing, not the text "None" (#3798)

BundleManifest.from_dict read every required scalar as
`str(raw.get(key, "")).strip()`. The `""` default only covers a MISSING key. A
key present but null -- exactly how YAML spells an empty field (`author:` with
nothing after it) -- yields None, and `str(None)` is the literal string "None".
That value is non-empty, so it sailed past the `if not value` required-field
checks in structural_errors().

Reproduced on main:

    bundle.yml with description:/author:/license: left empty
    -> description='None'  author='None'  license='None'
    -> structural_errors() == []
    -> specify bundle validate: exit 0, "demo is well-formed and valid."

So an empty required field was silently accepted and the bundle shipped the
literal text "None" as its author/license/description -- which is what
`bundle info` and a catalog entry then display. A null `provides.<kind>[].id`
likewise became a component literally named "None".

Add a `_text()` helper beside the existing `_parse_str_list` (the file's
established "one coercion helper applied at every site" shape) mapping an
explicit null to "", and route the required scalars through it. Same
silent-acceptance class as the already-merged guards in this function: #3629
(non-mapping `integration:`) and #3661 (falsy non-mapping requires/provides).

Non-null values are still `str()`-coerced and stripped, and an absent key
already produced "" -- so valid manifests are byte-for-byte unaffected.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/contract/test_manifest_schema.py

@@ -26,6 +26,45 @@ def test_missing_required_field_is_reported_by_name():
     assert any("bundle.license" in e for e in errors)
 
 
+@pytest.mark.parametrize(
+    "field", ["name", "role", "description", "author", "license"]
+)
+def test_explicit_null_bundle_field_is_reported_as_missing(field):
+    """A field present but null is how YAML spells an empty value (`author:`).
+
+    `str(None)` is the literal text "None", which is non-empty, so it passed the
+    required-field checks: the bundle validated clean and shipped "None" as its
+    author/license/description.
+    """
+    data = valid_manifest_dict()
+    data["bundle"][field] = None
+    manifest = BundleManifest.from_dict(data)
+    assert getattr(manifest.bundle, field) == ""
+    assert any(f"bundle.{field}" in e for e in manifest.structural_errors())
+
+
+def test_explicit_null_speckit_version_is_reported_as_missing():
+    data = valid_manifest_dict()
+    data["requires"]["speckit_version"] = None
+    manifest = BundleManifest.from_dict(data)
+    assert manifest.requires.speckit_version == ""
+    assert any("speckit_version" in e for e in manifest.structural_errors())
+
+
+def test_explicit_null_component_id_is_not_named_none():
+    """A null component id must not become a component literally named "None"."""
+    data = valid_manifest_dict()
+    for kind, items in (data.get("provides") or {}).items():
+        if isinstance(items, list) and items and isinstance(items[0], dict):
+            items[0]["id"] = None
+            break
+    else:  # pragma: no cover - fixture is expected to provide components
+        pytest.skip("fixture has no component list to null out")
+    manifest = BundleManifest.from_dict(data)
+    assert manifest.components, "fixture is expected to declare components"
+    assert all(ref.id != "None" for ref in manifest.components)
+
+
 def test_unsupported_schema_version_is_rejected():
     data = valid_manifest_dict(schema_version="9.9")
     errors = BundleManifest.from_dict(data).structural_errors()