[codex] Remove obsolete Iceberg code paths

[bill-ph]

Summary

• Remove obsolete Iceberg and Lakekeeper runtime, provisioning, control-plane, server, transpiler, docs, scripts, and test paths.
• Keep DuckLake as the only managed warehouse catalog, with explicit rejection for legacy Iceberg provisioning/admin/default-catalog inputs.
• Add a configstore migration to drop legacy Iceberg columns from existing stores.
• Clean up integration fixture isolation surfaced by the removal work.

Validation

• just lint
• just test-unit
• just test-controlplane
• just test-controlplane-k8s
• just test-configstore-integration
• just test-integration

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	39	26
E2E/journey files	0	4	0
Workflow files	0	1	0

Signals

• Test cases: +5 / -165
• Assertions: +31 / -750
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: likely reduced

Warnings

• Test files deleted (likely reduced)
  • controlplane/configstore/iceberg_backend_test.go
  • controlplane/lakekeeper_inputs_test.go
  • controlplane/provisioner/cleanup_helpers_test.go
  • controlplane/provisioner/controller_lakekeeper_test.go
  • controlplane/provisioner/lakekeeper_client_smoke_test.go
  • controlplane/provisioner/lakekeeper_client_test.go
  • controlplane/provisioner/lakekeeper_e2e_helpers_test.go
  • controlplane/provisioner/lakekeeper_e2e_test.go
  • ... and 18 more
• Test cases removed (likely reduced)
  • 165 removed vs 5 added
• Assertions removed (needs review)
  • 750 removed vs 31 added
• E2E or journey files changed (needs review)
  • tests/e2e-mw-dev/README.md
  • tests/e2e-mw-dev/harness.sh
  • tests/e2e-mw-dev/manifests.tmpl.yaml
  • tests/e2e-mw-dev/run.sh