Fix multicast TTL when updating SDP destinations

[garethsb]

Summary

• preserve an explicit multicast TTL when gst-nmos-rs overrides an SDP destination
• apply the gst-nmos-rs multicast default when an override changes a no-TTL destination to multicast, and clear TTL state for unicast
• default a zero configuring-SDP TTL before nvnmos regenerates a sender transport file after IS-05 activation

This fixes the zero-TTL behavior reported by @souravpaul98 when a caps-only nmossink starts with the unspecified 0.0.0.0 destination and IS-05 later assigns a multicast address.

Test plan

• cargo fmt -p gst-nmos-rs -- --check
• cargo test -p gst-nmos-rs --lib
• cargo test -p gst-nmos-rs --lib -- --test-threads=1
• build libnvnmos against the current nmos-cpp main revision
• end-to-end nvnmosd test: activate a sender from c=IN IP4 0.0.0.0 to multicast and verify /transportfile contains /32