Declarative agent prototype: Add OpenResponses streaming format support to AI SDK provider

[smurching]

Summary

Add OpenResponsesLanguageModel to the AI SDK provider to support agent apps that return OpenResponses SSE format.

This moves OpenResponses support from individual consumer apps (like e2e-chatbot-app-next) into the shared library, making it available to all TypeScript/JavaScript consumers.

Related Issue: Improves architecture for OpenResponses streaming support
Specification: https://github.com/databricks/openresponses

Changes

New Module: open-responses-language-model/

• open-responses-schema.ts - Zod schemas for event validation (text delta, item done, error)
• open-responses-api-types.ts - TypeScript types for request/response
• open-responses-convert-to-input.ts - AI SDK → OpenResponses format conversion
• open-responses-convert-to-message-parts.ts - OpenResponses → AI SDK stream parts
• open-responses-stream-transformer.ts - SSE stream parser and transformer
• open-responses-language-model.ts - Main LanguageModelV3 implementation

Updated Files

• databricks-provider.ts - Added openResponses() method
• index.ts - Export schemas, types, and utilities
• package.json - Bumped version to 0.5.0

Tests

• open-responses.test.ts - 10 comprehensive tests for schema validation and conversion
• All 248 tests passing

Architecture

Follows the same pattern as existing implementations (responses-agent-language-model, fmapi-language-model):

const provider = createDatabricksProvider({ baseURL, headers });
const model = provider.openResponses('agent-endpoint');
const result = await streamText({ model, messages });

Benefits

1. Reusable - Available to all TypeScript consumers, not just one app
2. Consistent - Follows established patterns in databricks-ai-bridge
3. Tested - Comprehensive test coverage
4. Maintainable - Centralized implementation reduces duplication

Testing

npm run build  # ✅ Build successful
npm test       # ✅ All 248 tests passing

Follow-up

After this PR merges, we can update consumer apps (like e2e-chatbot-app-next) to use @databricks/ai-sdk-provider@^0.5.0 and remove their embedded OpenResponses implementations.

Companion PR: databricks/app-templates@main...smurching:app-templates:feature/openresponses-support

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