bundle: add bundle.managed_state config field + DMS test server

Introduce the managed-state configuration gate that future commits will
use to swap deployment locking (and, later, resource state management)
between the historical workspace-filesystem implementation and the new
Deployment Metadata Service (DMS) implementation.

The gate mirrors the existing bundle.engine pattern:

  - bundle/config/managedstate/managedstate.go exposes the
    DATABRICKS_BUNDLE_MANAGED_STATE env var, a Default constant, and a
    Setting struct with location-aware source attribution.
  - config.Bundle.ManagedState (json:"managed_state") is the bundle
    config knob.
  - cmd/bundle/utils.ResolveManagedStateSetting layers config > env >
    Default with file/line source attribution.

Also lands the in-memory bundle deployment metadata service test
fixture (libs/testserver/deployment_metadata.go and route
registrations) so a follow-up that wires the real DMS-backed lock can
add its acceptance tests without further test-infra churn.

Acceptance coverage:
  - acceptance/bundle/managed-state/gate-config asserts the config
    field surfaces through bundle validate -o json.
  - acceptance/bundle/managed-state/gate-env documents that the env
    var deliberately does not mutate validated config (it is consumed
    lazily at deploy/destroy/bind/unbind time).

No behavior changes for existing bundles -- nothing yet consumes the
new setting.

Author: shreyas-goenka

acceptance/bundle/managed-state/gate-config/databricks.yml

@@ -0,0 +1,7 @@
+bundle:
+  name: test-managed-state-gate-config
+  managed_state: true
+
+targets:
+  default:
+    default: true

acceptance/bundle/managed-state/gate-config/out.test.toml

@@ -0,0 +1,3 @@
+
+>>> [CLI] bundle validate -o json
+true

acceptance/bundle/managed-state/gate-config/output.txt

@@ -0,0 +1 @@
+trace $CLI bundle validate -o json | jq '.bundle.managed_state'

acceptance/bundle/managed-state/gate-config/script

@@ -0,0 +1 @@
+Ignore = [".databricks"]

acceptance/bundle/managed-state/gate-config/test.toml

@@ -0,0 +1,6 @@
+bundle:
+  name: test-managed-state-gate-env
+
+targets:
+  default:
+    default: true

acceptance/bundle/managed-state/gate-env/databricks.yml

@@ -0,0 +1,9 @@
+
+>>> DATABRICKS_BUNDLE_MANAGED_STATE=true [CLI] bundle validate -o json
+null
+
+>>> DATABRICKS_BUNDLE_MANAGED_STATE=false [CLI] bundle validate -o json
+null
+
+>>> [CLI] bundle validate -o json
+null

acceptance/bundle/managed-state/gate-env/out.test.toml

@@ -0,0 +1,7 @@
+# The env var does not surface in bundle config dumps; it is consumed lazily
+# at deploy/destroy/bind/unbind time via ResolveManagedStateSetting. This test
+# pins that contract by showing the env var leaves .bundle.managed_state
+# unset in the validated config, regardless of the env var value.
+trace DATABRICKS_BUNDLE_MANAGED_STATE=true $CLI bundle validate -o json | jq '.bundle.managed_state'
+trace DATABRICKS_BUNDLE_MANAGED_STATE=false $CLI bundle validate -o json | jq '.bundle.managed_state'
+trace $CLI bundle validate -o json | jq '.bundle.managed_state'

acceptance/bundle/managed-state/gate-env/output.txt

@@ -0,0 +1 @@
+Ignore = [".databricks"]