chore: cp-12.18.0 - Remove MULTICHAIN_API feature flag and expose Multichain API on all build types (#32351)

Previously the Multichain API was only available on Flask and Beta
builds. Starting in 12.18.0 we are releasing the Multichain API to all
build types including `stable`

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/32351?quickstart=1)

## **Related issues**

Fixes: MetaMask/MetaMask-planning#4779

## **Manual testing steps**

1. Build this branch in a normal stable build (`yarn start`)
2. Go to https://metamask.github.io/test-dapp-multichain/latest/
3. Enter your extensionId (watch video below to see where) -> Detection
on `stable` blocked by MetaMask/providers#420
4. See that you're able to connect and interact with the multichain api 

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**


https://github.com/user-attachments/assets/282cfd24-428f-4eff-ab27-1a9424c27ad0

### **After**


https://github.com/user-attachments/assets/7af12e3c-05a8-4277-a439-5c54fefdd13d


## **Pre-merge author checklist**

- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

---------

Co-authored-by: jiexi <jiexiluan@gmail.com>

Author: adonesky1

app/manifest/v2/brave.json

@@ -1,7 +1,7 @@
 {
   "content_security_policy": "frame-ancestors 'none'; script-src 'self' 'wasm-unsafe-eval'; object-src 'none'; font-src 'self';",
   "externally_connectable": {
-    "matches": ["https://metamask.io/*"],
+    "matches": ["http://*/*", "https://*/*"],
     "ids": ["*"]
   }
 }

app/manifest/v2/chrome.json

@@ -1,7 +1,7 @@
 {
   "content_security_policy": "frame-ancestors 'none'; script-src 'self' 'wasm-unsafe-eval'; object-src 'none'; font-src 'self';",
   "externally_connectable": {
-    "matches": ["https://metamask.io/*"],
+    "matches": ["http://*/*", "https://*/*"],
     "ids": ["*"]
   },
   "minimum_chrome_version": "89"

app/manifest/v3/chrome.json

@@ -4,7 +4,7 @@
     "sandbox": "sandbox allow-scripts; script-src 'self' 'unsafe-inline' 'unsafe-eval'; object-src 'none'; default-src 'none'; connect-src *; font-src 'self';"
   },
   "externally_connectable": {
-    "matches": ["https://metamask.io/*"],
+    "matches": ["http://*/*", "https://*/*"],
     "ids": ["*"]
   },
   "minimum_chrome_version": "89"

app/scripts/background.js

@@ -1045,7 +1045,7 @@ export function setupController(
 
       connectEip1193(portStream, remotePort.sender);
 
-      if (process.env.MULTICHAIN_API && isFirefox) {
+      if (isFirefox) {
         const mux = setupMultiplex(portStream);
         mux.ignoreStream(METAMASK_EIP_1193_PROVIDER);
 
@@ -1062,7 +1062,7 @@ export function setupController(
       overrides?.getPortStream?.(remotePort) || new PortStream(remotePort);
 
     const isDappConnecting = remotePort.sender.tab?.id;
-    if (isDappConnecting && process.env.MULTICHAIN_API) {
+    if (isDappConnecting) {
       if (metamaskBlockedPorts.includes(remotePort.name)) {
         return;
       }
@@ -1084,10 +1084,6 @@ export function setupController(
   };
 
   connectCaipMultichain = (connectionStream, sender) => {
-    if (!process.env.MULTICHAIN_API) {
-      return;
-    }
-
     controller.setupUntrustedCommunicationCaip({
       connectionStream,
       sender,

app/scripts/inpage.js

@@ -38,15 +38,11 @@ import { initializeProvider } from '@metamask/providers/initializeInpageProvider
 import ObjectMultiplex from '@metamask/object-multiplex';
 import { pipeline } from 'readable-stream';
 
-// this is currently equivalent to process.env.MULTICHAIN_API
-// which can't be used for conditional imports
-///: BEGIN:ONLY_INCLUDE_IF(build-beta,build-flask)
 import {
   getMultichainClient,
   getDefaultTransport,
 } from '@metamask/multichain-api-client';
 import { registerSolanaWalletStandard } from '@metamask/solana-wallet-standard';
-///: END:ONLY_INCLUDE_IF
 
 import shouldInjectProvider from '../../shared/modules/provider-injection';
 import { METAMASK_EIP_1193_PROVIDER } from './constants/stream';
@@ -91,12 +87,9 @@ if (shouldInjectProvider()) {
     },
   });
 
-  // this is currently equivalent to process.env.MULTICHAIN_API
-  ///: BEGIN:ONLY_INCLUDE_IF(build-beta,build-flask)
   getMultichainClient({
     transport: getDefaultTransport(),
   }).then((client) => {
     registerSolanaWalletStandard({ client });
   });
-  ///: END:ONLY_INCLUDE_IF
 }