fix: Environment variable merging in StdioClientTransport

[edwarddjss]

Fix environment variable handling in StdioClientTransport by properly merging default and custom environments.

Motivation and Context

Currently when providing custom environment variables, the code uses the nullish coalescing operator (??) which either uses custom environment OR default environment, not both. This fix ensures default environment variables are always included while still allowing custom variables to override them.

How Has This Been Tested?

Updated the existing environment variable test and added a new test specifically for checking environment variable merging. All tests pass locally.

Breaking Changes

None. This is a bugfix that maintains the existing API.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This PR addresses issue #216.

[jspahrsummers]

The current behavior is deliberate and has been discussed before.