fix(bundler): reject falsy non-mapping requires/provides in manifest from_dict (#3661)

* fix(bundler): reject falsy non-mapping requires/provides in manifest from_dict

BundleManifest.from_dict used `data.get("requires") or {}` and
`data.get("provides") or {}`, so a FALSY non-mapping value ([], '', 0,
false) was coerced to {} BEFORE the isinstance guard — a malformed manifest
passed validation as one that requires/provides nothing. Only a truthy
non-mapping (e.g. "extensions") was rejected.

Handle None explicitly (default to {}) and reject every other non-mapping,
matching the sibling 'integration' guard added in #3629. Absent fields still
parse to the empty default.

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

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(bundler): correct absent-optional-mapping regression assertion

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/specify_cli/bundler/models/manifest.py

@@ -111,8 +111,10 @@ class BundleManifest:
             license=str(bundle_raw.get("license", "")).strip(),
         )
 
-        requires_raw = data.get("requires") or {}
-        if not isinstance(requires_raw, dict):
+        requires_raw = data.get("requires")
+        if requires_raw is None:
+            requires_raw = {}
+        elif not isinstance(requires_raw, dict):
             raise BundlerError("'requires' must be a mapping when present.")
         requires = Requires(
             speckit_version=str(requires_raw.get("speckit_version", "")).strip(),
@@ -130,8 +132,10 @@ class BundleManifest:
         if isinstance(integration_raw, dict) and integration_raw.get("id"):
             integration = IntegrationRef(id=str(integration_raw["id"]).strip())
 
-        provides = data.get("provides") or {}
-        if not isinstance(provides, dict):
+        provides = data.get("provides")
+        if provides is None:
+            provides = {}
+        elif not isinstance(provides, dict):
             raise BundlerError("'provides' must be a mapping when present.")
 
         tags_raw = data.get("tags")