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.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.