Add UpdateItem to database service for partial attribute updates

[NitinKumar004]

Summary

• Adds UpdateItem method to the database driver interface supporting SET (add/overwrite fields) and REMOVE (delete fields) actions
• Implements across all 3 providers: AWS DynamoDB, Azure CosmosDB, GCP Firestore
• Wires through portable API layer with metrics, recording, rate limiting, and error injection
• Emits stream/change feed events on update (MODIFY event with old/new images)
• Emits provider-native metrics (ConsumedWriteCapacityUnits, TotalRequestUnits, document/write_count)

Closes #58

Test plan

• TestUpdateItemAWS — SET + REMOVE flow with GetItem verification
• TestUpdateItemAzure — SET + REMOVE flow with GetItem verification
• TestUpdateItemGCP — SET + REMOVE flow with GetItem verification
• TestUpdateItemNotFound — all 3 providers return NotFound for missing items
• TestUpdateItemTableNotFound — all 3 providers return NotFound for missing tables
• TestUpdateItemInvalidAction — rejects unsupported action types
• TestUpdateItemWithStreams — MODIFY stream records emitted with old/new images
• Linter: 0 issues (golangci-lint run --timeout=9m ./...)
• Full test suite: all passing (go test ./...)