pipeline-codec: reset sched before encode alloc

The codec encode path allocated its graph without first resetting the
shared scheduler, unlike every sibling path (decode, prompt-builder,
speaker_encoder_extract). After a synthesis call leaves the codec sched
allocated, the next RVQ encode — e.g. registering a second cloned voice
after a generation — trips GGML_ASSERT(!sched->is_alloc) in
ggml_backend_sched_alloc_graph and crashes the server.

Reset the sched before alloc to match the other call sites.

Repro: register a voice (base model) -> generate speech -> register
another voice. Second register crashes without this fix.