Support to override the billing scope for the remote playback option

[pragmatrix]

For situations in which no billing scope is used, it's useful to override it, specifically for remote playbacks which may be billed depending what's being played back.

[Copilot]

Pull Request Overview

Adds support for an optional billing scope that can be overridden for remote playback, ensuring billing records can attribute charges correctly.

• Extend protocol and core types to carry billing_scope through events
• Update context switch and services to unpack and (partially) forward billing_scope
• Revise examples to initialize the new field

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/protocol.rs	Added billing_scope to ClientEvent::Text
src/context_switch.rs	Unpack and forward billing_scope in ClientEvent::Text handler
services/playback/src/lib.rs	Map billing_scope in playback service (currently hardcoded None)
services/azure/src/synthesize.rs	Updated pattern match to accept extra fields (..)
services/aristech/src/synthesize.rs	Simplified pattern match using ..
examples/azure-synthesize.rs	Initialize billing_scope: None
examples/aristech-synthesize.rs	Initialize billing_scope: None
core/src/conversation.rs	Added billing_scope to Input::Text

Comments suppressed due to low confidence (2)

core/src/conversation.rs:281

• [nitpick] Consider adding a doc comment for the new billing_scope field in Input::Text to explain its purpose and how consumers should use it.

 billing_scope: Option<String>, 

src/protocol.rs:51

• There are no tests covering the new billing_scope functionality. Consider adding unit or integration tests to verify that overrides flow correctly through the system.

 billing_scope: Option<String>,