feat: add context stats API

[varin-nair-factory]

Summary

• add typed ContextStats request/response support for droid.get_context_stats
• add DroidClient.getContextStats() and DroidSession.getContextStats()
• keep context stats pull-only for now and cover the new API in client/session/schema tests

Example

import { createSession } from '@factory/droid-sdk';

const session = await createSession({
  execPath: 'droid-dev',
  cwd: process.cwd(),
});

const stats = await session.getContextStats();
console.log(stats);
// { used, remaining, limit, accuracy, updatedAt }

await session.close();

Testing

• npm run typecheck
• npm run typecheck:examples
• npm test
• npm run lint
• npm run format:check
• npx --yes tsx examples/test-droid-dev.ts

Closes FAC-19074