Add section-scoped route listing to ui_list_routes

### Description

Replaces the original filtering approach in #7922 (closed) per the discussion on #7920.

Right now `ui_list_routes` returns the full route list on every call, which is a lot of payload for the LLM to reason over. Instead of free-text/fuzzy filtering, add a required `section` argument to `ui_list_routes`, validated against a closed enum (one value per feature area, matching `meta.section` added in #7921), and only return routes belonging to that section.

This keeps the payload small, and since `section` is a closed enum rather than a free string, it also prevents the LLM from requesting or hallucinating a section that doesn't exist.

### Epic/Story
Part of #7920
https://github.com/FlowFuse/engineering/issues/160

### Have you provided an initial effort estimate for this issue?
I have provided an initial effort estimate

Find the ui_list_routes implementation and the route metadata introduced in #7921; first inspect how routes are currently collected and how meta.section is represented. Define the allowed section values and verify that only matching routes are returned, with validation rejecting other values.

Written by the indexing model from the issue text.