fix: align data source create/update with 2025-09-03 database API

[lunaxislu]

Description

• Fix endpoint mapping for data source tools to match Notion API 2025-09-03
  behavior.
• create-a-data-source now maps to POST /v1/databases.
• update-a-data-source now maps to PATCH /v1/databases/{data_source_id}.
• Update create payload schema to require initial_data_source.properties
  instead of top-level properties.
• Add regression tests for endpoint mappings and create payload shape.
• Update README migration notes for the payload change.

Fixes #218

How was this change tested?

• Automated test (unit, integration, etc.)
• Manual test (provide reproducible testing steps below)

Automated:

• npm test -- src/openapi-mcp-server/openapi/__tests__/notion-openapi-data- source.test.ts
• npm test -- src/openapi-mcp-server/openapi/__tests__/parser.test.ts src/ openapi-mcp-server/mcp/__tests__/proxy.test.ts
• npm run build

Screenshots

N/A