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_browsing_test.go

@@ -288,7 +288,7 @@ var _ = Describe("Browsing Endpoints", func() {
 
 			Expect(resp.Status).To(Equal(responses.StatusOK))
 			Expect(resp.Genres).ToNot(BeNil())
-			Expect(resp.Genres.Genre).To(HaveLen(3))
+			Expect(resp.Genres.Genre).To(HaveLen(4))
 		})
 
 		It("includes correct genre names", func() {