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/tools/src/lib.rs

@@ -57,6 +57,7 @@ pub use responses_api::dynamic_tool_to_responses_api_tool;
 pub use responses_api::mcp_tool_to_deferred_responses_api_tool;
 pub use responses_api::mcp_tool_to_responses_api_tool;
 pub use responses_api::tool_definition_to_responses_api_tool;
+pub use tool_call::ConversationHistory;
 pub use tool_call::ToolCall;
 pub use tool_config::ShellCommandBackendConfig;
 pub use tool_config::ToolEnvironmentMode;