fix(librarian): replace nullable tool params with sentinel defaults

Ollama's OpenAI-compatible API mishandles anyOf[X, null] parameter
schemas. update_wiki_page (content/title/tags/description) and
smart_create_wiki_page (path) now use empty-string/empty-list
sentinels translated to None inside the tool, following the
biographer pattern from 9d7ce39.

Adds a snapshot test that walks every registered librarian tool's
emitted JSON schema and fails on any anyOf[..., null].

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/agents/librarian/tools.py

@@ -793,10 +793,10 @@ async def read_urls_batch(
 
 async def update_wiki_page(
     page_id: int,
-    content: str | None = None,
-    title: str | None = None,
-    tags: list[str] | None = None,
-    description: str | None = None,
+    content: str = "",
+    title: str = "",
+    tags: list[str] = [],  # noqa: B006 - sentinel, never mutated
+    description: str = "",
 ) -> str:
     """
     Update an existing wiki page.
@@ -810,12 +810,15 @@ async def update_wiki_page(
     - Updating tags to organize pages into dossiers
     - Fixing descriptions or titles
 
+    Note: empty values are sentinels for "leave unchanged" (Ollama's
+    OpenAI-compatible API mishandles anyOf[X, null] parameter schemas).
+
     Args:
         page_id: ID of the page to update (get from search_wiki results)
-        content: New markdown content (optional - only if changing content)
-        title: New title (optional - only if renaming)
-        tags: New tag list (optional - replaces existing tags)
-        description: New description (optional)
+        content: New markdown content (empty = leave unchanged)
+        title: New title (empty = leave unchanged)
+        tags: New tag list, replaces existing tags (empty = leave unchanged)
+        description: New description (empty = leave unchanged)
 
     Returns:
         Confirmation with updated page details
@@ -829,21 +832,21 @@ async def update_wiki_page(
         async with LibraryDeskClient() as client:
             page = await client.update_wiki_page(
                 page_id=page_id,
-                content=content,
-                title=title,
-                tags=tags,
-                description=description,
+                content=content if content else None,
+                title=title if title else None,
+                tags=tags if tags else None,
+                description=description if description else None,
             )
 
             # Build update summary
             updated_fields = []
-            if content is not None:
+            if content:
                 updated_fields.append("content")
-            if title is not None:
+            if title:
                 updated_fields.append("title")
-            if tags is not None:
+            if tags:
                 updated_fields.append("tags")
-            if description is not None:
+            if description:
                 updated_fields.append("description")
 
             output_parts = [
@@ -948,7 +951,7 @@ async def create_wiki_page(
 async def smart_create_wiki_page(
     topic: str,
     tags: list[str],
-    path: str | None = None,
+    path: str = "",
     include_web_research: bool = True,
     include_wiki_search: bool = True,
 ) -> str:
@@ -970,7 +973,7 @@ async def smart_create_wiki_page(
     Args:
         topic: The topic to research and create a page about
         tags: List of tags/dossiers for categorization
-        path: Optional custom path (auto-generated from topic if not provided)
+        path: Optional custom path (empty = auto-generated from topic)
         include_web_research: Whether to search the web (default: True)
         include_wiki_search: Whether to search existing wiki (default: True)
 
@@ -986,7 +989,7 @@ async def smart_create_wiki_page(
             response = await client.smart_create_wiki_page(
                 topic=topic,
                 tags=tags,
-                path=path,
+                path=path if path else None,
                 include_web_research=include_web_research,
                 include_wiki_search=include_wiki_search,
             )