Create output as Audio file

[paullizer]

Using speech to text to output to file

[paullizer]

Shipped in v0.250.102 via #1117.

Release note entries:

• v0.250.102: Per-Message Audio Export

Full notes: https://microsoft.github.io/simplechat/explanation/release_notes/

Closing as completed. This issue stayed open after the fix merged because pull requests target Development while the repository default branch is main, so GitHub never applied the Fixes # keyword automatically.