fix(slack): log state-backend failures during unfurl enrichment

Address gemini-code-assist review on PR #89 (line 1844). The previous
``except Exception: return links`` swallowed state-backend errors
silently, making it hard to debug why link enrichment isn't firing.
Log a warning with the exception and message_ts before returning the
fallback. Behavior unchanged otherwise — bare URLs still surface to
handlers when state reads fail.

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

Author: claude

src/chat_sdk/adapters/slack/adapter.py

@@ -1840,7 +1840,11 @@ async def _enrich_links(self, links: list[LinkPreview], message_ts: str | None)
         while True:
             try:
                 stored = await state.get(f"slack:unfurls:{message_ts}")
-            except Exception:
+            except Exception as exc:
+                self._logger.warn(
+                    "Failed to read unfurl data from state",
+                    {"error": str(exc), "message_ts": message_ts},
+                )
                 return links
             if stored or time.monotonic() >= deadline:
                 break