Fix stale file read after patch causing empty diff content

[SteffenDE]

Also ensure events are always handled in order.

#167 fixed some diff errors, but there was still a race condition when using codex in full access mode, where the file was already updated when codex-acp read it, making the patch fail to apply and returning empty content:

codex-acp/src/CodexToolCallMapper.ts

Lines 294 to 297 in e698674

	const oldContent = await readFileContent(change.path);
	if (oldContent !== null) {
	const patchedContent = applyPatch(oldContent, unifiedDiff);
	if (patchedContent === false) return null;

Additionally, because each event was processed individually, the await readFile could cause the item/completed event to be processed and sent as tool_call_update via ACP before the initial tool_call was sent.