Update FAR implementation to better support programmatic invocation

[Colengms]

Opening this to track potential work to make cpptools implementation of FAR a little more friendly for programmatic invocation, such as when called from an AI tool (copilot_listCodeUsages) or using #usages in Copilot chat.

Specifically, we might want to:

• Suppress the UI progress dialog when invoked programmatically. (Cancellation should be handled through whatever operation is invoking it programmatically).
• Ensure multiple requests can be queued. (We shouldn't assume there is a singular FAR 'session' that gets cancelled when a new request is received. It's possible AI tools may issue multiple requests in parallel.)