Deprecate undocumented custom server methods (#94348)
### What?
Deprecate undocumented custom-server methods exposed by `next()` while
preserving their runtime behavior.
- Add TypeScript deprecation annotations and one-time runtime warnings
for `setAssetPrefix()`, logging helpers, `revalidate()`, and legacy
`render*` helpers.
- Extend custom-server tests to cover warnings and warning
deduplication.
- Move unrelated test fixtures away from deprecated custom-server calls.
### Why?
These methods are not part of the documented custom-server API, but
existing applications may still rely on them. Warning-only deprecation
gives users migration guidance without introducing a breaking runtime
change.
### How?
Warnings are emitted only through `NextCustomServer`, so framework-owned
`NextServer` usage remains quiet. The warning text points `render*`
callers toward `app.getRequestHandler()`, `setAssetPrefix()` callers
toward configuration, and logging and revalidation callers toward
supported application APIs.
The tests retain compatibility assertions for existing behavior, add
deliberate invocations for previously uncovered methods, verify
once-per-process warning deduplication, and remove incidental warning
sources in unrelated fixtures.
<!-- NEXT_JS_LLM_PR -->
