Refine SIP session timer negotiation behavior

[yeoleobun]

Summary

• separate the basic session_timer switch from a new session_timer_always option
• align session timer negotiation and refresher selection with PJPROJECT behavior
• keep local refresh ownership in always mode when a successful refresh response omits Session-Expires

Why

The previous implementation mixed negotiated and forced session-timer behavior behind one flag, which made it hard to match PJPROJECT semantics. It also had a caller-leg bug in always mode: when the peer replied to a refresh without Session-Expires, the code forced the refresher to uac unconditionally, which could flip ownership to the remote side on the inbound leg and stop further local refreshes.

Impact

• session_timer=true keeps the normal negotiated behavior
• session_timer_always=true forces PJPROJECT-style always behavior without using Require: timer
• inbound and outbound legs now choose the refresher consistently with PJPROJECT
• the caller leg no longer stops refreshing after a header-less 200 OK in always mode

Validation

• cargo test test_session_timer_mode -- --nocapture
• cargo test session_timer -- --nocapture
• cargo test apply_refresh_response -- --nocapture
• cargo test test_init_callee_timer_disabled_without_session_expires -- --nocapture