Conditionally bypass cache when adding styles.

Working on Textualize/textual#1587 surfaced a caching issue that is best shown by running the code below:

```py
from rich.style import Style

meta = {"click": "something"}
s1 = Style.from_meta(meta)
s2 = Style.from_meta(meta)
print(s1.link_id, s2.link_id)  # Different link_id.

base = Style(color="red")
print((base + s1).link_id)  # This is the link_id of s1.
print((base + s2).link_id)  # This is the link_id of s1!
```

The change presented here will bypass cache when adding styles that have a link id but don't have a link attribute (if they did, so would the combined style and the call to .copy would refresh the link id either way).
Simply refreshing the link id will break Textual link highlighting.

rich/style.py

@@ -729,8 +729,7 @@ def test(self, text: Optional[str] = None) -> None:
         text = text or str(self)
         sys.stdout.write(f"{self.render(text)}\n")
 
-    @lru_cache(maxsize=1024)
-    def _add(self, style: Optional["Style"]) -> "Style":
+    def _add_no_cache(self, style: Optional["Style"]) -> "Style":
         if style is None or style._null:
             return self
         if self._null:
@@ -754,8 +753,20 @@ def _add(self, style: Optional["Style"]) -> "Style":
         new_style._hash = None
         return new_style
 
+    @lru_cache(maxsize=1024)
+    def _add(self, style: Optional["Style"]) -> "Style":
+        return self._add_no_cache(style)
+
     def __add__(self, style: Optional["Style"]) -> "Style":
-        combined_style = self._add(style)
+        if (
+            style is not None
+            and not style._link
+            and not self._link
+            and (self._link_id or style._link_id)
+        ):
+            combined_style = self._add_no_cache(style)
+        else:
+            combined_style = self._add(style)
         return combined_style.copy() if combined_style.link else combined_style