This pull request resolves #279.

This pull request fixes the lack of precise typing for OpenAI raw model stream events while preserving the released generic stream surface in @openai/agents-core. It adds OpenAI-specific narrowing helpers in @openai/agents-openai, threads a lightweight source marker through raw model events, and updates examples/docs to use the new guards instead of hand-written raw_model_stream_event checks and unsafe casts.

The main changes are:

• add source derivation to raw model stream events in packages/agents-core/src/events.ts
• add isOpenAIResponsesRawModelStreamEvent(...) and isOpenAIChatCompletionsRawModelStreamEvent(...) plus typed aliases in packages/agents-openai/src/rawModelEvents.ts, and export them through @openai/agents-openai and @openai/agents
• tag OpenAI Responses and Chat Completions raw model events with an explicit source in packages/agents-openai/src/openaiResponsesModel.ts and packages/agents-openai/src/openaiChatCompletionsStreaming.ts
• add regression coverage in packages/agents-core/test/events.test.ts and packages/agents-openai/test/rawModelEvents.test.ts, plus update existing OpenAI streaming tests
• update user-facing examples in examples/basic/stream-ws.ts, examples/connectors/index.ts, examples/tools/code-interpreter.ts, and examples/docs/mcp/hostedStream.ts to use the new helpers
• update the streaming guide in docs/src/content/docs/guides/streaming.mdx
• include an untracked new docs example file: examples/docs/streaming/openaiResponsesRawModelStreamEvent.ts

This change is additive rather than breaking: generic consumers can keep using RunRawModelStreamEvent, while OpenAI-specific consumers now get a sound way to recover the precise Responses or Chat Completions event unions.