feat: update McpClient::new_stdio_client() to accept an env (#831)

Cleans up the signature for `new_stdio_client()` to more closely mirror
how MCP servers are declared in config files (`command`, `args`, `env`).
Also takes a cue from Claude Code where the MCP server is launched with
a restricted `env` so that it only includes "safe" things like `USER`
and `PATH` (see the `create_env_for_mcp_server()` function introduced in
this PR for details) by default, as it is common for developers to have
sensitive API keys present in their environment that should only be
forwarded to the MCP server when the user has explicitly configured it
to do so.

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[//]: # (BEGIN SAPLING FOOTER)
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* #829
* __->__ #831

codex-rs/Cargo.lock

@@ -561,7 +561,6 @@ name = "codex-mcp-client"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "codex-core",
  "mcp-types",
  "pretty_assertions",
  "serde",