fix(langfuse): redact base64 data URIs instead of truncating into invalid base64

The Langfuse SDK treats `data:*;base64,...` strings as media and tries to
decode them. `_truncate_text` was slicing those strings mid-payload, producing
invalid base64 and noisy "Error parsing base64 data URI" logs. Observability
only needs the metadata, not raw image/audio bytes, so redact the whole data
URI (type, media_type, length) before it reaches the SDK.

Salvaged the Langfuse fix from #39682 onto current main as a standalone,
single-concern change (the dashboard `dist/**` and plugin-discovery parts of
that PR already landed separately on main).

Co-authored-by: foras910521-lab <foras910521-lab@users.noreply.github.com>

tests/plugins/test_langfuse_plugin.py

@@ -171,6 +171,40 @@ class TestHooksInert:
         mod.on_post_tool_call(tool_name="read_file", args={}, result="ok", task_id="t", session_id="s")
 
 
+class TestPayloadSanitization:
+    def test_safe_value_redacts_base64_data_uri_instead_of_truncating(self):
+        sys.modules.pop("plugins.observability.langfuse", None)
+        import importlib
+        mod = importlib.import_module("plugins.observability.langfuse")
+
+        payload = "data:image/png;base64," + ("a" * 20000)
+        result = mod._safe_value(payload)
+
+        assert result == {
+            "type": "data_uri",
+            "media_type": "image/png",
+            "omitted": True,
+            "length": len(payload),
+        }
+
+    def test_serialize_messages_redacts_data_uri_parts(self):
+        sys.modules.pop("plugins.observability.langfuse", None)
+        import importlib
+        mod = importlib.import_module("plugins.observability.langfuse")
+
+        payload = "data:image/jpeg;base64," + ("b" * 20000)
+        serialized = mod._serialize_messages([
+            {"role": "user", "content": [{"type": "image_url", "image_url": {"url": payload}}]}
+        ])
+
+        assert serialized[0]["content"][0]["image_url"]["url"] == {
+            "type": "data_uri",
+            "media_type": "image/jpeg",
+            "omitted": True,
+            "length": len(payload),
+        }
+
+
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 # Placeholder-credential guard (#23823).
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