refactor(rust): restrict test-only functions

check_from_deezer and the lyrics cache_size accessor are only called by
unit tests in their own crate, so compile them only for tests and narrow
them to the crate.

call_download is only used by the extensions integration tests, which
can only reach public items; the download manager goes through
call_download_provider. Keep it public but hide it from the docs.

RequestLease::wait_cancelled stays public: the extensions crate's unit
tests call it across the crate boundary and a later coalescer change is
expected to use it.

rust_backend/crates/extensions/src/runtime.rs

@@ -570,6 +570,9 @@ impl ExtensionRuntime {
 
     /// Native provider-manager entry point. Resolver/callback/retry time shares
     /// one allowance; established native media reads pause only that allowance.
+    // Public only for the integration tests in `tests/`; the download manager
+    // uses `call_download_provider`.
+    #[doc(hidden)]
     pub fn call_download(
         &self,
         method: &str,

rust_backend/crates/providers/src/lyrics/service.rs

@@ -162,7 +162,8 @@ impl LyricsService {
         Ok(())
     }
 
-    pub fn cache_size(&self) -> usize {
+    #[cfg(test)]
+    pub(crate) fn cache_size(&self) -> usize {
         self.inner.cache.len()
     }
 

rust_backend/crates/providers/src/resolver/availability/cache.rs

@@ -148,7 +148,8 @@ impl AvailabilityService {
         self.with_raw(check, |resolver, check| album(resolver, id, check))
     }
 
-    pub fn check_from_deezer(
+    #[cfg(test)]
+    pub(crate) fn check_from_deezer(
         &self,
         id: &str,
         check: &Check<'_>,