fix: add Milvus search result validation with actionable diagnostics

[afadhitya]

Closes #393

Summary

Add VectorSearchResultValidationError and validateMilvusSearchResultRow() to guard against null Milvus search results that caused search_code to fail with "Cannot read properties of null (reading 'scores')" even after successful indexing.

Changes

Core package

• search-result-validation.ts — new validation utility: VectorSearchResultValidationError error class + validateMilvusSearchResultRow() with actionable diagnostics
• milvus-vectordb.ts — validate result rows in both regular and hybrid search before mapping scores
• milvus-restful-vectordb.ts — same validation in RESTful client (both search + hybrid search)
• context.ts — log embedding provider, expected dimension, and actual query dimension during search to surface mismatches early
• vectordb/index.ts — export new validation symbols
• Tests:
  • search-result-validation.test.ts — 3 tests (null row, missing score, valid)
  • milvus-vectordb.search-validation.test.ts — 2 tests (null result, null score)
  • context.search-dimensions.test.ts — 1 test (dimension logging)

MCP package

• handlers.ts — catch VectorSearchResultValidationError and return actionable error message (dimension mismatch, collection mismatch, etc.) instead of generic "check if indexed"
• Tests:
  • handlers.search-validation.test.ts — 1 test (actionable diagnostics)

Verification

• All 34 core tests pass (9 suites)
• All 6 MCP tests pass