fix(mcp): preserve Windows env vars for MCP servers (#4180)

[cpjet64]

Fixes #4180.

What
Expand the Windows DEFAULT_ENV_VARS list so MCP servers inherit COMSPEC, SYSTEMROOT, PROGRAMDATA, APPDATA, and other core variables used by CLI helpers and DNS
add a Windows-only regression test (test_create_env_for_mcp_server_includes_windows_defaults) that guards each entry in that list
keep the existing override test working cross‑platform by reading USERNAME on Windows

Why
Without those inherited variables, Windows MCP servers that shell out (Docker MCP gateway, npx-based servers, etc.) fail immediately because plugins, caches, or system DLLs can’t be located. The new test ensures we don’t regress on the required variable set.

How
Update codex-rs/mcp-client/src/mcp_client.rs
Introduce EnvVarGuard to safely mutate/restore env vars during tests
Document the rationale in the inline comment

Testing
just fmt
cargo test -p codex-mcp-client
just fix -p codex-mcp-client

[cpjet64]

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