feat: payload index management

[jordanpartridge]

Summary

• Adds CreatePayloadIndexRequest and DeletePayloadIndexRequest
• Adds Qdrant::createPayloadIndex() and Qdrant::deletePayloadIndex()
• Supports all field types: keyword, integer, float, geo, text, bool, datetime, uuid
• Contract tests updated to validate against Qdrant OpenAPI spec

Closes #5

Usage

$qdrant->createPayloadIndex('music_events', 'artist', 'keyword');
$qdrant->createPayloadIndex('music_events', 'hour', 'integer');
$qdrant->deletePayloadIndex('music_events', 'artist');

Test plan

• 107 tests, 100% coverage
• Unit tests for both requests (endpoint, body, query params)
• Feature tests with mocks
• Contract tests for endpoint + schema validation