refactor: address code review feedback for auth and pagination improvements

- Extract duplicated thread/message pagination functions to shared utils module
- Refactor pagination token parsing to use Option combinators instead of defaults
- Split access token generation from assignment for clearer error handling
- Add appservice token collision detection at startup and registration
- Allow appservice re-registration with same token (for config updates)
- Simplify thread relation chunk building using iterator chaining
- Fix saturating_inc edge case in relation queries with explicit filtering
- Add concise comments explaining non-obvious behaviour choices

src/api/client/mod.rs

@@ -36,6 +36,7 @@ pub(super) mod typing;
 pub(super) mod unstable;
 pub(super) mod unversioned;
 pub(super) mod user_directory;
+pub(super) mod utils;
 pub(super) mod voip;
 pub(super) mod well_known;