Refactor: extract shared finalizeCall helper for internal and trunk 200 OK handling

[thorsager]

Background

Issue #41 extracts a callCtx struct to remove the long parameter lists in the B2BUA response loops. Once that is complete, handleTrunk200OK and handleBob200OK still contain nearly identical logic for:

• parsing Bob's SDP and extracting the RTP address,
• building and sending the ACK to Bob,
• constructing the 200 OK answer to Alice,
• creating media.Session, media.Bridge, sip.Dialog, and Call objects,
• starting the bridge (early vs delayed offer).

Goal

Extract a shared helper that finalizes a successful B2BUA call for both internal and trunk legs:

func (h *Handler) finalizeCall(ctx context.Context, cc *callCtx, resp *proto.SIPMessage, bobTo *proto.SIPAddress) *Call

The helper should accept the trunk-specific data it needs (e.g. trunkName, bobReqURI, sessionExpires) as additional parameters, or via optional fields in a small trunk-specific wrapper.

Acceptance criteria

• handleTrunk200OK and handleBob200OK delegate the common finalization to finalizeCall.
• No change in SIP or RTP behavior.
• make test and make lint pass.
• Blocked until Refactor: extract callCtx struct to eliminate long parameter lists in B2BUA response loops #41 is merged.