fix(outbound): send SIP CANCEL for early dialogs when closing

[michalrokita]

Summary

Previously, Close() only sent BYE for answered calls (when inviteOk != nil) but simply dropped state for ringing calls without sending any SIP signaling. This left the far-end device ringing indefinitely.

Problem

When an outbound call is terminated while in "ringing" state (early dialog - after INVITE sent but before final response received), the remote party's phone continues ringing because no SIP signaling was sent to cancel the call.

Solution

Per RFC 3261 Section 9.1, a UAC should send CANCEL to terminate an early dialog. This change:

• Adds sendCancel() method that creates CANCEL using sip.NewCancelRequest()
• CANCEL uses the same CSeq as the original INVITE (RFC requirement - NewCancelRequest handles this)
• Modifies Close() to call sendCancel() when invite exists but inviteOk is nil

Changes

• Added sendCancel(ctx context.Context) method to sipOutbound
• Modified Close() to detect early dialog state and send CANCEL appropriately

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 31.25000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.76%. Comparing base (0460b40) to head (e9ba07a).
⚠️ Report is 210 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/sip/outbound.go	31.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #567      +/-   ##
==========================================
- Coverage   65.25%   64.76%   -0.50%     
==========================================
  Files          51       34      -17     
  Lines        6588     6422     -166     
==========================================
- Hits         4299     4159     -140     
+ Misses       1915     1849      -66     
- Partials      374      414      +40     

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[dennwc]

Look good, thank you!