fix (ai/mcp): prevent mutation of customEnv

[cgoinglove]

Description

This PR fixes an issue where the original customEnv object passed to functions in get-environment.ts was being mutated due to JavaScript's pass-by-reference behavior for objects.

Problem

When a configuration object with an env property was passed to createChildProcess, any modifications to the environment inside the function would also affect the original object, potentially causing unexpected behavior in subsequent code.

In our project, we encountered this issue specifically when using serverConfig with the MCP client:

const serverConfig = await findMcpServerConfig(id);
// {
//   command: 'node server.js',
//   env: { PORT: '3000' }
// };

const client = await createMCPClient({
      name,
      onUncaughtError,
      transport: new StdioMCPTransport(serverConfig),
});

return Response.json(...)

After the createMCPClient call, we discovered that the original serverConfig.env was being modified. This could cause problems when:

1. The same config object is reused for multiple operations
2. The config needs to be stored back to the database in its original form
3. The config is used for comparison or validation later in the code

This change preserves the immutability of configuration objects throughout the application, making the code more predictable and preventing hard-to-trace bugs that might occur when objects are unexpectedly modified.

[lgrammel]

1. please add a patch changeset for ai with the PR title as description
2. please add tests for get-environment.ts that reproduce the issue

[cgoinglove]

@lgrammel Thanks for your review!

Let's gooo 🚀