feat(tern): support Vitess pull schema

[aparajon]

Why

SchemaBot onboarding can already pull MySQL schema state, but Vitess databases need the same control-plane initiated path so keyspace schemas can be made declarative.

What

• Enable PullSchema execution for Vitess database routes
• Pull Vitess keyspace schemas through the existing PlanetScale/Vitess client path
• Return each keyspace as a namespace with table DDL and vschema.json artifacts
• Let schemabot onboard materialize Vitess namespaces and VSchema artifacts

Risk Assessment

Medium — this extends the pull/onboard path for Vitess, but leaves apply behavior unchanged and only runs when callers explicitly request a Vitess pull.

Generated with Amp

[Copilot]

Pull request overview

This PR extends SchemaBot’s existing “pull live schema → materialize declarative files” onboarding path to support Vitess databases via the existing PlanetScale/Vitess client API, including exporting vschema.json as a namespace artifact.

Changes:

• Allow PullSchema routing/execution for Vitess targets and fetch keyspace schemas + vschema.json via the PlanetScale client.
• Update schemabot onboard to accept Vitess pull responses and write vschema.json artifacts to disk.
• Add unit + integration coverage for Vitess PullSchema behavior and API routing.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/tern/local_client.go	Adds Vitess PullSchema support using PlanetScale client calls (keyspace discovery, schema + VSchema fetch).
pkg/tern/local_client_test.go	Adds unit tests for Vitess PullSchema (single keyspace + discovery path) using a mock PS client.
pkg/tern/local_client_integration_test.go	Adds integration test that pulls live Vitess schema from a LocalScale container.
pkg/cmd/commands/onboard.go	Extends onboard write plan to support Vitess and write vschema.json artifacts.
pkg/cmd/commands/onboard_test.go	Adds test verifying Vitess keyspace artifacts are written during onboarding.
pkg/api/plan_handlers.go	Allows PullSchema execution for Vitess targets (and updates error messaging accordingly).
pkg/api/handlers_test.go	Adds API test for configured Vitess PullSchema routing; updates unsupported-type test to use a different type.

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