fix: resolve Windows MCP server execution for script-based tools (#3828)

## What?

Fixes MCP server initialization failures on Windows when using
script-based tools like `npx`, `pnpm`, and `yarn` that rely on
`.cmd`/`.bat` files rather than `.exe` binaries.

Fixes #2945

## Why?

Windows users encounter "program not found" errors when configuring MCP
servers with commands like `npx` in their `~/.codex/config.toml`. This
happens because:

- Tools like `npx` are batch scripts (`npx.cmd`) on Windows, not
executable binaries
- Rust's `std::process::Command` bypasses the shell and cannot execute
these scripts directly
- The Windows shell normally handles this by checking `PATHEXT` for
executable extensions

Without this fix, Windows users must specify full paths or add `.cmd`
extensions manually, which breaks cross-platform compatibility.

## How?

Added platform-specific program resolution using the `which` crate to
find the correct executable path:

- **Windows**: Resolves programs through PATH/PATHEXT to find
`.cmd`/`.bat` scripts
- **Unix**: Returns the program unchanged (no-op, as Unix handles
scripts natively)

### Changes

- Added `which = "6"` dependency to `mcp-client/Cargo.toml`
- Implemented `program_resolver` module in `mcp_client.rs` with
platform-specific resolution
- Added comprehensive tests for both Windows and Unix behavior

### Testing

Added platform-specific tests to verify:
- Unix systems execute scripts without extensions
- Windows fails without proper extensions
- Windows succeeds with explicit extensions
- Cross-platform resolution enables successful execution

**Tested on:**
- Windows 11 (NT 10.0.26100.0 x64)
- PowerShell 5.1 & 7+, CMD, Git Bash
- MCP servers: playwright, context7, supabase
- WSL (verified no regression)

**Local checks passed:**
```bash
cargo test && cargo clippy --tests && cargo fmt -- --config imports_granularity=Item
```

### Results

**Before:**
```
🖐 MCP client for `playwright` failed to start: program not found
```

**After:**
```
🖐 MCP client for `playwright` failed to start: request timed out
```

Windows users can now use simple commands like `npx` in their config
without specifying full paths or extensions. The timeout issue is a
separate concern that will be addressed in a follow-up PR.

---------

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/rmcp-client/src/rmcp_client.rs

@@ -47,6 +47,7 @@ use crate::logging_client_handler::LoggingClientHandler;
 use crate::oauth::OAuthCredentialsStoreMode;
 use crate::oauth::OAuthPersistor;
 use crate::oauth::StoredOAuthTokens;
+use crate::program_resolver;
 use crate::utils::apply_default_headers;
 use crate::utils::build_default_headers;
 use crate::utils::convert_call_tool_result;
@@ -91,13 +92,20 @@ impl RmcpClient {
         cwd: Option<PathBuf>,
     ) -> io::Result<Self> {
         let program_name = program.to_string_lossy().into_owned();
-        let mut command = Command::new(&program);
+
+        // Build environment for program resolution and subprocess
+        let envs = create_env_for_mcp_server(env, env_vars);
+
+        // Resolve program to executable path (platform-specific)
+        let resolved_program = program_resolver::resolve(program, &envs)?;
+
+        let mut command = Command::new(resolved_program);
         command
             .kill_on_drop(true)
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .env_clear()
-            .envs(create_env_for_mcp_server(env, env_vars))
+            .envs(envs)
             .args(&args);
         if let Some(cwd) = cwd {
             command.current_dir(cwd);