Return read errors with VStream schema guidance

[nickvanw]

Summary

• return non-timeout read errors instead of logging a warning and reporting sync success
• preserve the read-attempt start cursor when row/update serialization callbacks fail, so failed work does not advance state
• detect Vitess/VStream schema incompatibility errors while building a table replication plan after schema changes
• add a specific historical re-sync instruction and surface that schema case as gRPC FailedPrecondition
• add a real PlanetScale integration regression for mid-table DDL requiring historical re-sync

Closes #77.
Supersedes #78 and #87.

Notes

This intentionally does not auto-reset state or perform a historical re-sync. If the VStream schema incompatibility condition is hit, continuing incremental replication cannot make progress safely and the user/operator should trigger a Fivetran historical re-sync. Generic non-timeout read errors now fail the sync as well, preserving the underlying error instead of silently dropping it.

Tests

• env GOCACHE=/tmp/fivetran-go-build-cache go test ./...
• env GOCACHE=/tmp/fivetran-go-build-cache DATABASE_HOST=... DATABASE_NAME=... DATABASE_USERNAME=... DATABASE_PASSWORD=... go test -tags=integration ./cmd/internal/server/handlers -run TestIntegrationMidTableDDLRequiresHistoricalResync -count=1 -v
• env GOCACHE=/tmp/fivetran-go-build-cache DATABASE_HOST=... DATABASE_NAME=... DATABASE_USERNAME=... DATABASE_PASSWORD=... make test-integration-stress