feat: surface parent message ids in streams

[varin-nair-factory]

Description

Stacked on #44.

Adds parentMessageId to DroidUserMessage and DroidAssistantMessage, preserving the parent id already present on create-message notifications / messages instead of dropping it during stream conversion.

Also updates examples that consume assistant text deltas to opt into partial stream events so example typechecking stays aligned with the stream overloads introduced by #44.

Related Issue

N/A

Potential Risk & Impact

• Additive runtime/type field on user and assistant stream events.
• Existing stream consumers continue to work; consumers that manually construct these event objects may optionally include parentMessageId.
• Example updates are type-only behavior alignment: examples that read text deltas now explicitly request partial messages.

How Has This Been Tested?

• npx prettier --check src/stream.ts tests/stream.test.ts examples/fork-session.ts examples/interrupt-session.ts examples/multi-turn-session.ts examples/sdk-mcp-tool.ts
• npm run lint
• npm run typecheck
• npm run typecheck:examples
• npm test