fix: type safety improvements and PR #49 feedback

[mattapperson]

Summary

• Address PR Add callModel API with multiple consumption patterns #49 review feedback with improved type safety and code patterns
• Fix embeddings types in models and tests
• Update CI to use pnpm instead of npm
• Add Biome configuration for consistent linting

Changes

Type Safety Improvements

• Add type guard isNonStreamingResponse for safer response type checking
• Refactor IIFE patterns in getMessage/getText to private helper methods (getMessageInternal/getTextInternal)
• Add validation before assigning finalResponse
• Fix embeddings types to match API response structure

Code Quality

• Add braces to single-line if statements for readability
• Implement cleaner inline casts in stream handling

CI/Build

• Switch to pnpm for package management
• Add pnpm setup to Speakeasy workflow
• Remove redundant test typecheck step (handled by build and vitest)

Test plan

• Existing tests pass
• Type checking passes with pnpm exec tsc
• Linting passes with pnpm exec biome check