fix(realtime): raise WebRTC data-channel max-message-size + keep sendLoop alive

[localai-bot]

What

Raise the SCTP max-message-size LocalAI honors for the realtime oai-events
data channel, so larger server-generated events can be sent instead of being
silently dropped.

Why

In the WebRTC realtime transport, audio goes over an RTP track and events go
over the oai-events data channel (core/http/endpoints/openai/realtime_transport_webrtc.go,
t.dc.SendText(...)). Browsers advertise a conservative a=max-message-size
in their SDP offer — Chrome uses 262144 (256 KiB). pion enforces the remote's
advertised value on send, so a single realtime event larger than 256 KiB
cannot be sent: SendText fails, the event is dropped, the sendLoop exits,
and the turn yields no response to the client.

Some turns legitimately produce a single JSON event above 256 KiB — notably
tool-call turns (sizeable tool schemas/results). The failure is silent from
the client's side: transcription completes, then nothing comes back.

Browsers advertise the value conservatively, but their SCTP stacks reassemble
much larger messages in practice. So we raise the max-message-size honored for
our own server-generated events by rewriting the attribute in the offer
before SetRemoteDescription.

How

Two commits:

1. Raise the negotiated max-message-size. New
   raiseDataChannelMaxMessageSize(sdp) rewrites a=max-message-size:<n> in the
   offer to realtimeDataChannelMaxMessageSize (16 MiB) before
   pc.SetRemoteDescription. Offers without the attribute are unchanged.
   Self-contained (regexp/fmt only); applied at one call site in
   RealtimeCalls.

2. Don't tear down the session on a single failed send. sendLoop exited
   the sender goroutine on any SendText error, so one dropped event silently
   killed every subsequent event for the session. It now logs and skips the
   offending event (and keeps draining), while a genuinely dead transport is
   still handled by the <-t.closed / connection-state path. This is
   defense-in-depth: even with (1), an over-limit event degrades to a single
   dropped turn instead of a dead session.

Testing

• Unit specs for the helper (realtime_webrtc_sctp_test.go): raises a present
  value, leaves an attribute-less offer unchanged, rewrites every occurrence,
  and asserts the ceiling is above the 256 KiB browsers advertise.
• Reproduced end-to-end downstream: a realtime voice tool call failed with
  data channel send failed: outbound packet larger than maximum message size: 262144, producing no reply; raising the advertised size let the event through
  and the turn completed normally.

Notes

• 16 MiB is a generous-but-bounded ceiling (pion still fragments; these are
  trusted, server-generated events). Could be made configurable if preferred.
• Discovered while building a realtime voice app against LocalAI; the same
  failure mode affects any client whose browser advertises the 256 KiB default.