Enable plan tool by default (#5384)

## Summary
- make the plan tool available by default by removing the feature flag
and always registering the handler
- drop plan-tool CLI and API toggles across the exec, TUI, MCP server,
and app server code paths
- update tests and configs to reflect the always-on plan tool and guard
workspace restriction tests against env leakage

## Testing
Manually tested the extension. 
------
https://chatgpt.com/codex/tasks/task_i_68f67a3ff2d083209562a773f814c1f9

codex-rs/mcp-server/src/codex_tool_config.rs

@@ -49,10 +49,6 @@ pub struct CodexToolCallParam {
     /// The set of instructions to use instead of the default ones.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub base_instructions: Option<String>,
-
-    /// Whether to include the plan tool in the conversation.
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub include_plan_tool: Option<bool>,
 }
 
 /// Custom enum mirroring [`AskForApproval`], but has an extra dependency on
@@ -145,7 +141,6 @@ impl CodexToolCallParam {
             sandbox,
             config: cli_overrides,
             base_instructions,
-            include_plan_tool,
         } = self;
 
         // Build the `ConfigOverrides` recognized by codex-core.
@@ -159,7 +154,6 @@ impl CodexToolCallParam {
             model_provider: None,
             codex_linux_sandbox_exe,
             base_instructions,
-            include_plan_tool,
             include_apply_patch_tool: None,
             include_view_image_tool: None,
             show_raw_agent_reasoning: None,
@@ -277,10 +271,6 @@ mod tests {
                 "description": "Working directory for the session. If relative, it is resolved against the server process's current working directory.",
                 "type": "string"
               },
-              "include-plan-tool": {
-                "description": "Whether to include the plan tool in the conversation.",
-                "type": "boolean"
-              },
               "model": {
                 "description": "Optional override for the model name (e.g. \"o3\", \"o4-mini\").",
                 "type": "string"