test(e2e): add transcode endpoint e2e tests and clean up test helpers

Add comprehensive e2e tests for getTranscodeDecision and
getTranscodeStream endpoints covering direct play, transcoding,
error handling, and round-trip token validation. Refactor
buildPostReq to reuse buildReq for auth params, remove unused
WAV/AAC test tracks, and consolidate duplicate test assertions.

server/e2e/subsonic_multilibrary_test.go

@@ -141,7 +141,7 @@ var _ = Describe("Multi-Library Support", Ordered, func() {
 			resp := doReqWithUser(adminWithLibs, "getAlbumList", "type", "alphabeticalByName", "musicFolderId", fmt.Sprintf("%d", lib.ID))
 
 			Expect(resp.AlbumList).ToNot(BeNil())
-			Expect(resp.AlbumList.Album).To(HaveLen(6))
+			Expect(resp.AlbumList.Album).To(HaveLen(7))
 			for _, a := range resp.AlbumList.Album {
 				Expect(a.Title).ToNot(Equal("Symphony No. 9"))
 			}