Refine codec negotiation and telephone-event/8000 passthrough

[yeoleobun]

Summary

• preserve caller codec ordering when building the outbound offer to leg B
• prefer the codec selected by leg B when generating the answer for leg A
• preserve telephone-event/8000 separately from normal audio codec ordering and keep bridge-side DTMF PT rewriting

Details

• build leg B offer from caller codecs first, filtered by allow_codecs, then append remaining allowed codecs
• avoid dynamic PT conflicts that could hide Opus from leg B
• derive leg B codec from the callee answer instead of re-ranking answer codecs by PBX preference
• derive leg A codec by preferring leg B's selected codec when available, otherwise falling back to caller-side ordering
• preserve caller telephone-event/8000 in the offer to leg B and use negotiated DTMF PTs when creating the media bridge

Scope

• fixes unnecessary transcoding caused by allow_codecs-driven reordering
• fixes Opus loss caused by dynamic PT conflicts when appending extra codecs
• minimally supports telephone-event/8000 passthrough and PT rewriting
• does not add support for telephone-event/48000 or broader mid-call re-INVITE/UPDATE renegotiation changes