fix(openai-compatible,provider-utils): JSON schema validation

[dixoxib]

Background

OpenAI-compatible providers with strict JSON schema validation (like DeepSeek) reject all tool calls with the error:
Invalid schema for function: schema must be a JSON Schema of 'type: "object"', got 'type: null'.

This issue (#7924) affects all OpenAI-compatible providers when tools have no explicit inputSchema or when asSchema() creates empty schemas without the required type: "object" field.

Summary

Fixed JSON schema validation by:

1. @ai-sdk/provider-utils: Added missing type: 'object' to empty JSON schemas in asSchema() function
2. @ai-sdk/openai-compatible: Made prepareTools() async and added jsonSchema() fallback for undefined inputSchema
3. @ai-sdk/alibaba: Added await to prepareTools() call (required now that it's async)

These changes ensure that all generated JSON schemas are valid according to JSON Schema Draft 7 specification, which requires the type field.

Manual Verification

Verified with real DeepSeek API requests:

• Tool calling with DeepSeek no longer fails with "type: null" schema validation errors
• Tools with explicit schemas continue to work correctly
• Tools without explicit schemas now receive valid {type: 'object', properties: {}, additionalProperties: false} schemas
• All existing OpenAI-compatible provider tests pass (177 tests)
• Manual check for providers utilizing prepareTools()

Test commands executed:

# Build affected packages
cd packages/provider-utils && pnpm build
cd packages/openai-compatible && pnpm build

# Run tests
cd packages/openai-compatible && pnpm test  # All 177 tests pass

## Related Issues
Fixes #7924