feat(ivr): support route-point handoffs and empty prompts

[dengxiayehu]

Summary

This PR improves provider-driven IVR execution across routed application handoffs.

• Resolve route-point transfers through the configured call-routing pipeline and start the matched
  application.
• Preserve invocation-specific callee, SIP headers, variables, and execution identity across
  application handoffs.
• Keep provider session identity separate from route-produced business variables.
• Apply SIP header updates case-insensitively to avoid duplicate logical headers.
• Terminate unresolved or unsupported handoffs and release acquired routing resources, while
  retaining the configured direct IVR fallback behavior.
• Complete explicitly empty Prompt actions without waiting for an audio event, while preserving
  normal provider completion and trace reporting.
• Continue treating prompts with missing audio input as errors.

Verification

Regression coverage includes:

• Explicitly empty prompts and missing-media prompts.
• Route-point resolution and application startup.
• SIP header and route-variable propagation.
• Invocation identity across successive application generations.
• Route fallback, abort, unsupported-result, and startup-failure paths.