fix(controlplane): force DuckLake transpile mode in multitenant remote backend

[fuziontech]

Summary

• Multitenant control plane has empty server.Config.DuckLake.MetadataStore (per-org stores live in configstore), so the transpiler was running with DuckLakeMode: false. The DDL transform never registered, and Fivetran's ALTER TABLE … ADD PRIMARY KEY ("id") flowed straight through to the worker, which rejected it with Not implemented Error: Adding indexes or constraints is not supported in DuckLake.
• Add server.Config.AlwaysDuckLake and set it from the remote-backend control plane so DuckLake-aware transforms (DDL stripping, ON CONFLICT, pg_catalog/information_schema rewrites) run on every session.
• Extend TestTranspile_DDL_NoOps with the bare ADD PRIMARY KEY / ADD UNIQUE / ADD FOREIGN KEY forms; the existing case only covered the named-constraint variant.

Why this only bit MTCP and not single-tenant

Single-tenant deployments set DUCKGRES_DUCKLAKE_METADATA_STORE globally, so DuckLakeMode was already true. MTCP doesn't (each org has its own store), so the gate was always false there.

Test plan

• go test ./transpiler/... ./server/... ./controlplane/...
• go build ./... and go build -tags kubernetes ./...
• Deploy to a staging MTCP and re-run a Fivetran sync against posthog_data_import — confirm ALTER TABLE … ADD PRIMARY KEY now no-ops instead of erroring.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a multitenant control-plane issue where DuckLake-specific SQL transpilation was not being enabled (because DuckLake.MetadataStore is intentionally empty globally in MTCP), causing unsupported DDL like ALTER TABLE … ADD PRIMARY KEY to reach DuckDB/DuckLake workers and fail.

Changes:

• Add server.Config.AlwaysDuckLake to force DuckLake transpiler mode even when the global metadata store is empty.
• Enable AlwaysDuckLake automatically in the control plane when worker_backend=remote.
• Extend TestTranspile_DDL_NoOps to cover additional ALTER TABLE ... ADD {PRIMARY KEY|UNIQUE|FOREIGN KEY} forms (including the Fivetran-prefixed variant).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
transpiler/transpiler_test.go	Expands DDL no-op test cases to include bare constraint-add forms seen in real client traffic.
server/server.go	Introduces AlwaysDuckLake configuration flag on the server config struct.
server/conn.go	Forces transpiler DuckLake mode when either a metadata store is configured or AlwaysDuckLake is set.
controlplane/control.go	Sets AlwaysDuckLake=true automatically for remote (multitenant) worker backend mode.

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