Expose conversation history to extension tools (#23963)

## Why

Extension tools that need conversation context should be able to read it
from the live tool invocation instead of reaching into thread
persistence themselves.

## What changed

- Add a `ConversationHistory` snapshot to extension `ToolCall`s and
populate it from the current raw in-memory response history.
- Expose all history items at this boundary so each extension can filter
and bound the subset it needs before consuming or forwarding it.
- Cover the adapter and registry dispatch paths and update existing
extension tests that construct `ToolCall` literals.

## Test plan

- `cargo test -p codex-tools`
- `cargo test -p codex-extension-api`
- `cargo test -p codex-goal-extension`
- `cargo test -p codex-memories-extension`
- `cargo test -p codex-core passes_turn_fields_to_extension_call`
- `cargo test -p codex-core
extension_tool_executors_are_model_visible_and_dispatchable`

codex-rs/core/src/context_manager/history.rs

@@ -126,6 +126,11 @@ impl ContextManager {
         &self.items
     }
 
+    /// Returns raw items in the history and consumes the snapshot.
+    pub(crate) fn into_raw_items(self) -> Vec<ResponseItem> {
+        self.items
+    }
+
     pub(crate) fn history_version(&self) -> u64 {
         self.history_version
     }