fix(windows) shell_command on windows, minor parsing (#6811)

## Summary
Enables shell_command for windows users, and starts adding some basic
command parsing here, to at least remove powershell prefixes. We'll
follow this up with command parsing but I wanted to land this change
separately with some basic UX.

**NOTE**: This implementation parses bash and powershell on both
platforms. In theory this is possible, since you can use git bash on
windows or powershell on linux. In practice, this may not be worth the
complexity of supporting, so I don't feel strongly about the current
approach vs. platform-specific branching.

## Testing
- [x] Added a bunch of tests 
- [x] Ran on both windows and os x

codex-rs/core/tests/suite/model_tools.rs

@@ -119,7 +119,12 @@ async fn model_selects_expected_tools() {
     assert_eq!(
         gpt5_codex_tools,
         vec![
-            "shell".to_string(),
+            if cfg!(windows) {
+                "shell_command"
+            } else {
+                "shell"
+            }
+            .to_string(),
             "list_mcp_resources".to_string(),
             "list_mcp_resource_templates".to_string(),
             "read_mcp_resource".to_string(),
@@ -133,7 +138,12 @@ async fn model_selects_expected_tools() {
     assert_eq!(
         gpt51_codex_tools,
         vec![
-            "shell".to_string(),
+            if cfg!(windows) {
+                "shell_command"
+            } else {
+                "shell"
+            }
+            .to_string(),
             "list_mcp_resources".to_string(),
             "list_mcp_resource_templates".to_string(),
             "read_mcp_resource".to_string(),