chore: configure the 'maintenanceMode' flag to accept variants (#10956)

Configure the `maintenanceMode` flag type to be `boolean | Variant` and
update the env parsing to allow passing strings from the env.

The [first
impl](3bbfc9e681)
required you to set a full, variant -- stringified as json -- in the
env, but this is both error-prone and not very user friendly.
Additionally, the name of the variant isn't really important, and if
you're passing a string, you probably want it to be true.

As such, the [second
impl](c38357baa4)
updates the env parsing to read the full string value into a
pre-formatted variant if it's not parseable as a boolean.

As such, to set a custom message, you can now do:

```sh
UNLEASH_EXPERIMENTAL_MAINTENANCE_MODE='Custom message from plain env var string' yarn dev
```

With the [updates to the
UI](https://github.com/Unleash/unleash/pull/10961), it'll look a little
something like this:

<img width="388" height="64" alt="image"
src="https://github.com/user-attachments/assets/6b8a174b-d75f-4748-8f1a-1ad4ebce2073"
/>

## Rationale

This allows locking down Unleash instances with a custom message.
Previously, you'd have to use both maintenance mode and a custom banner
for this, but that requires more work to set properly and it shows two
banners, when you really only want the one.

frontend/src/interfaces/uiConfig.ts

@@ -52,7 +52,7 @@ export type UiFlags = {
     T?: boolean;
     UNLEASH_CLOUD?: boolean;
     UG?: boolean;
-    maintenanceMode?: boolean;
+    maintenanceMode?: boolean | Variant;
     messageBanner?: Variant;
     banner?: Variant;
     notifications?: boolean;

src/lib/types/experimental.ts

@@ -1,6 +1,9 @@
 import { PayloadType, type Variant } from 'unleash-client';
-import { parseEnvVarBoolean } from '../util/index.js';
 import { defaultVariant } from 'unleash-client/lib/variant.js';
+import {
+    parseEnvVarBoolean,
+    parseEnvVarBooleanOrStringVariant,
+} from '../util/index.js';
 import type { MetricFlagContext } from 'unleash-client/lib/impact-metrics/metric-types.js';
 import type { Context } from '../features/playground/feature-evaluator/index.js';
 
@@ -72,7 +75,7 @@ const flags: IFlags = {
         process.env.UNLEASH_RESPONSE_TIME_WITH_APP_NAME_KILL_SWITCH,
         false,
     ),
-    maintenanceMode: parseEnvVarBoolean(
+    maintenanceMode: parseEnvVarBooleanOrStringVariant(
         process.env.UNLEASH_EXPERIMENTAL_MAINTENANCE_MODE,
         false,
     ),

src/lib/ui-config/ui-config-service.ts

@@ -108,7 +108,6 @@ export class UiConfigService {
             email: user.email,
             userId: user.id,
         };
-
         const uiConfig: UiConfigSchema = {
             ...this.config.ui,
             flags,

src/lib/util/parseEnvVar.test.ts

@@ -1,5 +1,7 @@
+import { PayloadType } from 'unleash-client';
 import {
     parseEnvVarBoolean,
+    parseEnvVarBooleanOrStringVariant,
     parseEnvVarNumber,
     parseEnvVarStrings,
 } from './parseEnvVar.js';
@@ -39,3 +41,32 @@ test('parseEnvVarStringList', () => {
     expect(parseEnvVarStrings('a,b,c', [])).toEqual(['a', 'b', 'c']);
     expect(parseEnvVarStrings(' a,,,b,  c , ,', [])).toEqual(['a', 'b', 'c']);
 });
+
+test('parseEnvVarBooleanOrStringVariant', () => {
+    expect(parseEnvVarBooleanOrStringVariant(undefined, true)).toEqual(true);
+    expect(parseEnvVarBooleanOrStringVariant(undefined, false)).toEqual(false);
+    for (const truthy of ['true', 't', '1']) {
+        expect(parseEnvVarBooleanOrStringVariant(truthy, false)).toEqual(true);
+    }
+    for (const falsy of ['false', 'f', '0']) {
+        expect(parseEnvVarBooleanOrStringVariant(falsy, true)).toEqual(false);
+    }
+
+    expect(
+        parseEnvVarBooleanOrStringVariant(undefined, {
+            name: 'default-variant',
+            enabled: false,
+        }),
+    ).toEqual({ name: 'default-variant', enabled: false });
+
+    expect(
+        parseEnvVarBooleanOrStringVariant('custom string', true),
+    ).toMatchObject({
+        name: expect.any(String),
+        enabled: true,
+        payload: {
+            value: 'custom string',
+            type: PayloadType.STRING,
+        },
+    });
+});

src/lib/util/parseEnvVar.ts

@@ -1,3 +1,5 @@
+import { PayloadType, type Variant } from 'unleash-client';
+
 export function parseEnvVarNumber(
     envVar: string | undefined,
     defaultVal: number,
@@ -53,3 +55,29 @@ export function parseEnvVarJSON(
 
     return defaultVal;
 }
+
+export function parseEnvVarBooleanOrStringVariant(
+    envVar: string | undefined,
+    defaultVal: boolean | Variant,
+): boolean | Variant {
+    if (!envVar) {
+        return defaultVal;
+    }
+
+    if (envVar === '1' || envVar === 't' || envVar === 'true') {
+        return true;
+    }
+
+    if (envVar === '0' || envVar === 'f' || envVar === 'false') {
+        return false;
+    }
+
+    return {
+        name: 'Variant',
+        enabled: true,
+        payload: {
+            type: PayloadType.STRING,
+            value: envVar,
+        },
+    };
+}