fix(agent/tools): PubMed tool always returns "Unknown Authors" (#16330)

### What problem does this PR solve?

Fixes the PubMed tool always emitting `Authors: Unknown Authors`. The
`safe_find` closure in `_format_pubmed_content` was hardcoded to search
from the article root, so the per-author `LastName`/`ForeName` lookups
never matched.

`safe_find` now accepts an optional `base` node (defaults to `child`,
preserving the existing field lookups), and the author loop passes the
current `<Author>` element.

Closes #16328

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Add test cases

### Testing

Added `test/testcases/test_web_api/test_canvas_app/test_pubmed_unit.py`
covering per-author parsing, intact title/journal/DOI fields, and the
no-authors fallback.

Before: `Authors: Unknown Authors`
After:  `Authors: Furqan Khan, Jane Smith`

agent/tools/pubmed.py

@@ -117,8 +117,8 @@ class PubMed(ToolBase, ABC):
 
     def _format_pubmed_content(self, child):
         """Extract structured reference info from PubMed XML"""
-        def safe_find(path):
-            node = child
+        def safe_find(path, base=None):
+            node = child if base is None else base
             for p in path.split("/"):
                 if node is None:
                     return None
@@ -135,8 +135,8 @@ class PubMed(ToolBase, ABC):
         # Authors
         authors = []
         for author in child.findall(".//AuthorList/Author"):
-            lastname = safe_find("LastName") or ""
-            forename = safe_find("ForeName") or ""
+            lastname = safe_find("LastName", author) or ""
+            forename = safe_find("ForeName", author) or ""
             fullname = f"{forename} {lastname}".strip()
             if fullname:
                 authors.append(fullname)