feat: add `CODE_FUNCTION_NAME` to sentry-conventions, set to route handler function name

### Description

Re: https://github.com/getsentry/sentry-javascript/pull/23614#discussion_r3896481634

@Lms24 suggests:

> we might wanna look into if we should add something like a `{{code.function.name}}` template to the request handler name [rules](https://getsentry.github.io/sentry-conventions/names/#web_server-request-handler). But if we do, we can tackle it as a follow up

- Update sentry-conventions to make `CODE_FUNCTION_NAME` a template to optionally hold the request handler function name.
- Update request handlers to follow the convention.

Start by locating the sentry-conventions request-handler naming rules and the request-handler implementations in this repository. Update the convention so CODE_FUNCTION_NAME can optionally contain the handler function name, then bring the request handlers into compliance and verify the relevant checks pass.

Written by the indexing model from the issue text.