polish HTTP 304 docs/test/handling (#4129)

After #4120 was merged, here's a bit of polish on top.

- docs: Optimize JSDoc and comments.
- test: The existing 304 test is moved to the dedicated status code
block and extended with an `errorSpy` to also verify that no `error`
event is emitted.
- refactor: The success branch is moved before the error branch in
`fetch()`. This is more intuitive.

The bottom line is that this PR improves maintainability. For users,
this PR doesn't change anything.

Author: KristjanESPERANTO

js/http_fetcher.js

@@ -32,7 +32,7 @@ const ERROR_TYPE_TO_TRANSLATION = {
  * - Authentication support (Basic, Bearer)
  * - Self-signed certificate support
  * @augments EventEmitter
- * @fires HTTPFetcher#response - When fetch succeeds with ok response
+ * @fires HTTPFetcher#response - When fetch succeeds (including 304 Not Modified)
  * @fires HTTPFetcher#error - When fetch fails or returns non-ok response
  * @example
  * const fetcher = new HTTPFetcher(url, { reloadInterval: 60000 });
@@ -295,21 +295,21 @@ class HTTPFetcher extends EventEmitter {
 
 			const isSuccessfulResponse = response.ok || response.status === 304;
 
-			if (!isSuccessfulResponse) {
-				const { delay, errorInfo } = this.#getDelayForResponse(response);
-				nextDelay = delay;
-				this.emit("error", errorInfo);
-			} else {
+			if (isSuccessfulResponse) {
 				// Reset error counts on success
 				this.serverErrorCount = 0;
 				this.networkErrorCount = 0;
 
 				/**
-				 * Response event - fired when fetch succeeds
+				 * Response event - fired when fetch succeeds (including 304)
 				 * @event HTTPFetcher#response
 				 * @type {Response}
 				 */
 				this.emit("response", response);
+			} else {
+				const { delay, errorInfo } = this.#getDelayForResponse(response);
+				nextDelay = delay;
+				this.emit("error", errorInfo);
 			}
 		} catch (error) {
 			const isTimeout = error.name === "AbortError";

tests/unit/functions/http_fetcher_spec.js

@@ -51,31 +51,6 @@ describe("HTTPFetcher", () => {
 			expect(text).toBe(responseData);
 		});
 
-		it("should treat 304 responses as successful and reset error counters", async () => {
-			server.use(
-				http.get(TEST_URL, () => {
-					return new HttpResponse(null, { status: 304 });
-				})
-			);
-
-			fetcher = new HTTPFetcher(TEST_URL, { reloadInterval: 60000 });
-			fetcher.serverErrorCount = 2;
-			fetcher.networkErrorCount = 3;
-
-			const responsePromise = new Promise((resolve) => {
-				fetcher.on("response", (response) => {
-					resolve(response);
-				});
-			});
-
-			fetcher.startPeriodicFetch();
-			const response = await responsePromise;
-
-			expect(response.status).toBe(304);
-			expect(fetcher.serverErrorCount).toBe(0);
-			expect(fetcher.networkErrorCount).toBe(0);
-		});
-
 		it("should emit error event on network failure", async () => {
 			server.use(
 				http.get(TEST_URL, () => {
@@ -126,6 +101,34 @@ describe("HTTPFetcher", () => {
 	});
 
 	describe("HTTPFetcher - HTTP status code handling", () => {
+		describe("304 Not Modified", () => {
+			it("should emit response event for 304 and not emit error", async () => {
+				server.use(
+					http.get(TEST_URL, () => {
+						return new HttpResponse(null, { status: 304 });
+					})
+				);
+
+				fetcher = new HTTPFetcher(TEST_URL, { reloadInterval: 60000 });
+				fetcher.serverErrorCount = 2;
+				fetcher.networkErrorCount = 3;
+
+				const responsePromise = new Promise((resolve) => {
+					fetcher.on("response", resolve);
+				});
+				const errorSpy = vi.fn();
+				fetcher.on("error", errorSpy);
+
+				fetcher.startPeriodicFetch();
+				const response = await responsePromise;
+
+				expect(response.status).toBe(304);
+				expect(errorSpy).not.toHaveBeenCalled();
+				expect(fetcher.serverErrorCount).toBe(0);
+				expect(fetcher.networkErrorCount).toBe(0);
+			});
+		});
+
 		describe("401/403 errors (Auth failures)", () => {
 			it("should emit error with AUTH_FAILURE for 401", async () => {
 				server.use(