Add goal extension GoalApi (#25096)

## Summary

- add an extension-owned `GoalApi` for thread goal get/set/clear
operations
- register live goal runtimes with the API from the goal extension
backend
- cover the API and runtime-effect paths in goal extension tests

## Stack

Follow-up app-server wiring PR: #25108

## Validation

- `just fmt`
- `just fix -p codex-goal-extension`
- `just test -p codex-goal-extension`

codex-rs/ext/goal/src/tool.rs

@@ -363,7 +363,7 @@ where
         .map_err(|err| FunctionCallError::RespondToModel(err.to_string()))
 }
 
-fn validate_goal_budget(value: Option<i64>) -> Result<(), String> {
+pub(crate) fn validate_goal_budget(value: Option<i64>) -> Result<(), String> {
     if let Some(value) = value
         && value <= 0
     {
@@ -402,7 +402,7 @@ impl GoalToolResponse {
     }
 }
 
-async fn fill_empty_thread_preview_if_possible(
+pub(crate) async fn fill_empty_thread_preview_if_possible(
     state_db: &codex_state::StateRuntime,
     thread_id: ThreadId,
     goal: &codex_state::ThreadGoal,
@@ -441,7 +441,9 @@ fn protocol_status_from_state(status: codex_state::ThreadGoalStatus) -> ThreadGo
     }
 }
 
-fn state_status_from_protocol(status: ThreadGoalStatus) -> codex_state::ThreadGoalStatus {
+pub(crate) fn state_status_from_protocol(
+    status: ThreadGoalStatus,
+) -> codex_state::ThreadGoalStatus {
     match status {
         ThreadGoalStatus::Active => codex_state::ThreadGoalStatus::Active,
         ThreadGoalStatus::Paused => codex_state::ThreadGoalStatus::Paused,