fix(types): make RetrievalResult.score number | null to match API null return

[EricNGOntos]

Summary

Aligns the Node.js SDK's RetrievalResult interface with the updated API contract.

Background

The API's agentic retrieval route (/v1/retrieval/query) now returns null for score on chunks that were found only through KG navigation (no BM25 discovery score available). Previously the API hardcoded 1.0 as a placeholder, which was misleading.

Change

// Before
score: number;

// After
score: number | null;

null means 'no score available'. Consumers should treat it as unranked rather than assuming high relevance.

Testing

• All 240 existing tests pass: vitest run ✅
• TypeScript compilation succeeds: tsc --noEmit ✅
• Test fixtures using numeric scores (e.g. score: 1) remain valid since number is still accepted