fix: substitute every ${VAR} reference when merging env files (#1829)

## Problem

`mergeEnvVariables` expands `${VAR}` references in env file values (the
`python.envFile` file, the project `.env` and API overrides) with
`String.prototype.replace` and a string pattern. That has two effects:

- Only the first reference to a variable is replaced. With
`ROOT=/home/user` in the base environment, `PATHS=${ROOT}/a:${ROOT}/b`
becomes `/home/user/a:${ROOT}/b`.
- `$` sequences in the substituted value are read as replacement
patterns. A base value of `pa$$word` comes out as `pa$word`.

## Fix

Use `split(token).join(value)`, which replaces every occurrence and
inserts the value as is. (`replaceAll` is not available with the ES2020
lib target.)

## Tests

Added `src/test/features/execution/envVarUtils.unit.test.ts`, which
merges a value with a repeated `${ROOT}` reference and a `${SECRET}`
whose base value contains `$$`. It fails before the fix (actual
`/home/user/a:${ROOT}/b`) and passes after it.

- `npm run unittest` (Windows, Node 24): 2430 passing, 4 pending.
- eslint and prettier pass on the changed files.

Author: kwy404

src/features/execution/envVarUtils.ts

@@ -17,7 +17,7 @@ export function mergeEnvVariables(
             Object.keys(base).forEach((baseKey) => {
                 const baseValue = base[baseKey];
                 if (baseValue) {
-                    value = value?.replace(`\${${baseKey}}`, baseValue);
+                    value = value?.split(`\${${baseKey}}`).join(baseValue);
                 }
             });
             env[otherKey] = value;

src/test/features/execution/envVarUtils.unit.test.ts

@@ -0,0 +1,13 @@
+import * as assert from 'assert';
+import { mergeEnvVariables } from '../../../features/execution/envVarUtils';
+
+suite('Env Var Utils Tests', () => {
+    test('mergeEnvVariables substitutes every reference to a base variable', () => {
+        const env = mergeEnvVariables(
+            { ROOT: '/home/user', SECRET: 'pa$$word' },
+            { A: '${ROOT}/a:${ROOT}/b', B: '${SECRET}' },
+        );
+        assert.strictEqual(env.A, '/home/user/a:/home/user/b');
+        assert.strictEqual(env.B, 'pa$$word');
+    });
+});