fix(wellness): harden Form/TSB and date rendering from code review

Address findings from the pre-merge review:
- Form (TSB) now computes only when ctl/atl are numeric (isinstance guard),
  so a non-numeric value can no longer raise out of format_wellness_entry and
  take down the entire wellness render.
- The Date line uses `or` chaining so a present-but-null `date` falls back to
  `id` instead of rendering "Date: None".
- update_wellness injects the written date into the API echo when it lacks
  id/date, so the confirmation body can't read "Date: N/A" under a dated header.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NGzHtDvJur9U7ysgRKRUTN

src/intervals_mcp_server/tools/wellness.py

@@ -180,6 +180,10 @@ async def update_wellness(  # pylint: disable=too-many-arguments,too-many-positi
         return f"Error updating wellness data: {result.get('message')}"
 
     # Intervals.icu echoes back the full updated record; render it for confirmation.
+    # If the echo omits the date, inject the one we wrote to so the confirmation
+    # body doesn't read "Date: N/A" under a dated header.
     if isinstance(result, dict):
+        if not result.get("id") and not result.get("date"):
+            result["date"] = date
         return f"Updated wellness for {date}:\n\n" + format_wellness_entry(result)
     return f"Updated wellness for {date}."