test(coding-agent): covered docs.rs gunzip cap via extracted seam

- Extracted gunzipRustdocJson() with overridable maxOutputLength so the cap contract is testable with real gzip payloads instead of the banned mock.module().

packages/coding-agent/src/web/scrapers/docs-rs.ts

@@ -278,6 +278,17 @@ function findItemInModule(mod_: RustdocItem, name: string, index: Record<string,
 
 const DOCS_RS_CACHE_FILENAME = "rustdoc.json";
 
+/** Hard ceiling for decompressed rustdoc JSON: a 50 MB compressed payload can
+ *  expand far enough to block the event loop or OOM without a cap. Exceeding it
+ *  throws (RangeError), which the fetch path converts to a `null` result. */
+export const MAX_RUSTDOC_GUNZIP_BYTES = 256 * 1024 * 1024;
+
+/** Decompress a docs.rs rustdoc gzip payload with the output-size cap applied.
+ *  `maxOutputLength` is overridable only for tests exercising the cap contract. */
+export function gunzipRustdocJson(compressed: Buffer, maxOutputLength: number = MAX_RUSTDOC_GUNZIP_BYTES): string {
+	return gunzipSync(compressed, { maxOutputLength }).toString("utf-8");
+}
+
 function sanitizeCacheSegment(value: string): string {
 	return value.replace(/[^A-Za-z0-9._-]+/g, "_");
 }
@@ -399,7 +410,7 @@ export const handleDocsRs: SpecialHandler = async (
 		}
 
 		const compressed = Buffer.concat(chunks);
-		const jsonStr = gunzipSync(compressed, { maxOutputLength: 256 * 1024 * 1024 }).toString("utf-8");
+		const jsonStr = gunzipRustdocJson(compressed);
 		crate_ = tryParseJson<RustdocCrate>(jsonStr);
 		if (crate_?.index) {
 			await writeCachedRustdocCrate(target, jsonStr);

packages/coding-agent/test/web-scrapers/docs-rs-gunzip-cap.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, test } from "bun:test";
+import { gzipSync } from "node:zlib";
+import { gunzipRustdocJson, MAX_RUSTDOC_GUNZIP_BYTES } from "../../src/web/scrapers/docs-rs";
+
+describe("docs.rs rustdoc gunzip cap", () => {
+	test("decompresses payloads under the cap", () => {
+		const json = JSON.stringify({ root: "0", index: { "0": { name: "demo" } } });
+		expect(gunzipRustdocJson(gzipSync(json))).toBe(json);
+	});
+
+	test("rejects payloads whose decompressed size exceeds the cap", () => {
+		// A tiny compressed body expanding past the (test-scaled) cap must throw,
+		// which handleDocsRs converts into a null result instead of parsing.
+		const oversized = gzipSync("x".repeat(4096));
+		expect(() => gunzipRustdocJson(oversized, 1024)).toThrow(RangeError);
+	});
+
+	test("default cap is 256 MiB", () => {
+		expect(MAX_RUSTDOC_GUNZIP_BYTES).toBe(256 * 1024 * 1024);
+	});
+});