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>