feat: add includeRawChunks support for streaming

[robert-j-y]

Description

Implements includeRawChunks support for streaming calls in both chat and completion models. When includeRawChunks: true is passed to doStream(), the provider emits { type: 'raw', rawValue: <parsed chunk> } stream parts for each SSE event, giving consumers access to the raw provider chunks alongside the processed AI SDK stream parts.

This follows the same implementation pattern used in the Vercel AI SDK's @ai-sdk/openai-compatible provider - raw chunks are emitted at the start of the transform function, before any other processing.

Before:

const { stream } = await model.doStream({ prompt, includeRawChunks: true });
// Raw chunks not available

After:

const { stream } = await model.doStream({ prompt, includeRawChunks: true });
// Stream now includes { type: 'raw', rawValue: { id: '...', choices: [...], ... } } parts

Closes #340

Updates since last revision

Added test coverage for failed parse scenarios to document the intentional behavior: raw chunks are emitted before validation, which is useful for debugging malformed responses. When parsing fails, consumers receive both the raw chunk (containing the unparseable data) and the error chunk.

Human Review Checklist

• Verify raw chunk emission placement (before error handling) matches expected AI SDK behavior — Confirmed: matches the Vercel AI SDK @ai-sdk/openai-compatible reference implementation
• Confirm chunk.rawValue from ParseResult contains the expected parsed JSON — Confirmed: rawValue is always available on ParseResult (both success and failure cases)
• Verify behavior when parsing fails — Documented: new test confirms raw chunk is emitted before error handling, useful for debugging

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

---

Link to Devin run: https://app.devin.ai/sessions/f147987c7c8344f8a0f287947c44a709
Requested by: Robert Yeakel (@robert-j-y)