fix(realtime): Include noAction function in prompt template and handle tool_choice (#8372)

The realtime endpoint was not passing the noAction "answer" function to the
model in the prompt template, causing the model to always call user-provided
tools even when a direct response was appropriate.

Root cause:
- User tools were added to the funcs list
- TemplateMessages() was called to generate the prompt
- noAction function was only added AFTER templating
- This meant the prompt didn't include the "answer" function, even though
  the grammar did

Fix:
- Move noAction function creation before TemplateMessages() call so it's
  included in both the prompt and grammar
- Add proper tool_choice parameter handling to support "auto", "required",
  "none", and specific function selection
- Match behavior of the standard chat endpoint

💘 Generated with Crush

Assisted-by: Claude Sonnet 4.5 via Crush <crush@charm.land>

Signed-off-by: Richard Palethorpe <io@richiejp.com>