test(webpack-cli): add config format coverage for json, json5, yaml and toml

Add test cases covering data-style config formats so the planned move away
from `interpret` (towards Node.js' built-in loaders/`import`) cannot silently
change which formats are accepted.

- `json`: asserts JSON configs load (Node.js loads JSON natively), both via
  the `-c` flag and default config resolution.
- `json5`, `yaml`, `yml`, `toml`: regression tests pinning the current
  behavior. These extensions are listed by `interpret` but are absent from
  `interpret.jsVariants`, which is what webpack-cli uses to gate
  `interpret`/`rechoir`, so they are not loadable today and fail with exit
  code 2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013AushhUmU7F2dKWRE94XCg

Author: claude

test/build/config-format/json/json.test.js

@@ -0,0 +1,24 @@
+const { existsSync } = require("node:fs");
+const { resolve } = require("node:path");
+
+const { run } = require("../../../utils/test-utils");
+
+describe("webpack cli", () => {
+  it("should support JSON file as flag", async () => {
+    const { exitCode, stderr, stdout } = await run(__dirname, ["-c", "webpack.config.json"]);
+
+    expect(exitCode).toBe(0);
+    expect(stderr).toBeFalsy();
+    expect(stdout).toBeTruthy();
+    expect(existsSync(resolve(__dirname, "dist/foo.bundle.js"))).toBeTruthy();
+  });
+
+  it("should load JSON file by default", async () => {
+    const { exitCode, stderr, stdout } = await run(__dirname, []);
+
+    expect(exitCode).toBe(0);
+    expect(stderr).toBeFalsy();
+    expect(stdout).toBeTruthy();
+    expect(existsSync(resolve(__dirname, "dist/foo.bundle.js"))).toBeTruthy();
+  });
+});

test/build/config-format/json/main.js

@@ -0,0 +1 @@
+console.log("Hoshiumi Shoyo");

test/build/config-format/json/webpack.config.json

@@ -0,0 +1,7 @@
+{
+  "mode": "development",
+  "entry": "./main.js",
+  "output": {
+    "filename": "foo.bundle.js"
+  }
+}

test/build/config-format/json5/json5.test.js

@@ -0,0 +1,22 @@
+const { run } = require("../../../utils/test-utils");
+
+// Regression test for an unsupported config format.
+//
+// `interpret` lists `.json5` (via `json5/lib/register`), but webpack-cli only
+// runs `interpret`/`rechoir` for extensions present in `interpret.jsVariants`,
+// which contains JS variants only (`.ts`, `.coffee`, `.babel.js`, ...) and not
+// data formats such as `.json5`, `.yaml` or `.toml`. As a result a `.json5`
+// config is not loadable today, even when `json5` is installed.
+//
+// This test locks in that behavior so the planned move away from `interpret`
+// (towards Node.js' built-in loaders/`import`) does not silently change which
+// formats are accepted without a deliberate decision.
+describe("webpack cli", () => {
+  it("should not support JSON5 file as flag", async () => {
+    const { exitCode, stderr, stdout } = await run(__dirname, ["-c", "webpack.config.json5"]);
+
+    expect(exitCode).toBe(2);
+    expect(stderr).toContain("Failed to load 'webpack.config.json5' config");
+    expect(stdout).toBeFalsy();
+  });
+});

test/build/config-format/json5/main.js

@@ -0,0 +1 @@
+console.log("Kageyama Tobio");

test/build/config-format/json5/webpack.config.json5

@@ -0,0 +1,8 @@
+{
+  // JSON5 allows comments, unquoted keys and trailing commas
+  mode: "development",
+  entry: "./main.js",
+  output: {
+    filename: "foo.bundle.js",
+  },
+}

test/build/config-format/toml/main.js

@@ -0,0 +1 @@
+console.log("Tanjiro");

test/build/config-format/toml/toml.test.js

@@ -0,0 +1,22 @@
+const { run } = require("../../../utils/test-utils");
+
+// Regression test for an unsupported config format.
+//
+// `interpret` lists `.toml` (via `toml-require`), but webpack-cli only runs
+// `interpret`/`rechoir` for extensions present in `interpret.jsVariants`, which
+// contains JS variants only (`.ts`, `.coffee`, `.babel.js`, ...) and not data
+// formats such as `.toml`, `.yaml` or `.json5`. As a result a `.toml` config is
+// not loadable today.
+//
+// This test locks in that behavior so the planned move away from `interpret`
+// (towards Node.js' built-in loaders/`import`) does not silently change which
+// formats are accepted without a deliberate decision.
+describe("webpack cli", () => {
+  it("should not support TOML file as flag", async () => {
+    const { exitCode, stderr, stdout } = await run(__dirname, ["-c", "webpack.config.toml"]);
+
+    expect(exitCode).toBe(2);
+    expect(stderr).toContain("Failed to load 'webpack.config.toml' config");
+    expect(stdout).toBeFalsy();
+  });
+});

test/build/config-format/toml/webpack.config.toml

@@ -0,0 +1,5 @@
+mode = "development"
+entry = "./main.js"
+
+[output]
+filename = "foo.bundle.js"

test/build/config-format/yaml/main.js

@@ -0,0 +1 @@
+console.log("Nishinoya Yu");