fix(traces): cap captured body size to keep admin Traces UI responsive (#9946)

The trace middleware buffered the full request and response bodies for every
JSON exchange. With a chatty agent-pool RAG workload, /embeddings responses
(large vector arrays) accumulated to tens of MB in the in-memory buffer; the
admin Traces page would then download and parse 40+ MB on every load and on
every 5s auto-refresh, locking the UI in a loading state.

Add LOCALAI_TRACING_MAX_BODY_BYTES (default 64 KiB) that caps each captured
body. The full payload still flows through to the real client; only the
trace copy is bounded. Exchanges record body_truncated and original
body_bytes so the dashboard can show that truncation happened. The cap is
configurable via env, CLI, and runtime_settings.json.

Also unblock recovery: the Traces page now keeps the Clear button enabled
while loading, since "buffer too large to render" is exactly when the user
needs to clear it.


Assisted-by: Claude:claude-opus-4-7

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

core/http/react-ui/src/pages/Traces.jsx

@@ -406,7 +406,15 @@ export default function Traces() {
         <button className="btn btn-secondary btn-sm" onClick={fetchTraces}><i className="fas fa-rotate" /> Refresh</button>
         <button className="btn btn-secondary btn-sm" onClick={handleExport} disabled={traces.length === 0}><i className="fas fa-download" /> Export</button>
         <div style={{ flex: 1 }} />
-        <button className="btn btn-danger btn-sm" onClick={handleClear} disabled={traces.length === 0}><i className="fas fa-trash" /> Clear</button>
+        <button
+          className="btn btn-danger btn-sm"
+          onClick={handleClear}
+          /* Stay enabled while loading: a massive in-memory trace buffer is
+             precisely the case where the user can't see the table yet and
+             needs Clear to recover. Clearing an already-empty server-side
+             buffer is a harmless no-op. */
+          disabled={!loading && traces.length === 0}
+        ><i className="fas fa-trash" /> Clear</button>
       </div>
 
       {settings && (() => {