test: fix environment variable tests to verify actual values

[jonpspri]

Summary

Fixes environment variable tests in test_translate_stdio_endpoint.py that were not properly verifying that environment variables were actually set in the subprocess.

Changes

• Added _read_output() helper method to read subprocess output via pubsub subscription
• Updated test script to accept environment variable names as command line arguments (instead of via stdin)
• Rewrote 6 tests to read and verify actual environment variable values from subprocess output:
  • test_start_with_initial_env_vars
  • test_start_with_additional_env_vars
  • test_environment_variable_override
  • test_multiple_env_vars
  • test_env_vars_with_special_characters
  • test_large_env_vars
• Removed unnecessary os.environ.copy() in test_multiple_env_vars

Test Plan

uv run pytest tests/unit/mcpgateway/test_translate_stdio_endpoint.py -v

All 21 tests pass successfully.

Impact

Tests now properly validate that environment variables are correctly passed to and set in the subprocess, rather than just checking that the process started.

[Copilot]

Pull Request Overview

This PR fixes environment variable tests in test_translate_stdio_endpoint.py that were previously not verifying that environment variables were actually set in the subprocess. The tests now read and validate actual subprocess output instead of just checking process startup.

• Added a _read_output() helper method to reliably read subprocess output via pubsub subscription
• Modified the test script to accept environment variable names as command line arguments
• Rewrote 6 tests to verify actual environment variable values from subprocess output

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[jonpspri]

@crivetimihai I inadvertently addressed the root cause of all those PR StdioIOEndpoint test failures we're seeing in the PR checks. LMK if I need to isolate it, otherwise, I'm lining up a bunch of PRs behind this one.