Allow optional Street Level provider composition

src/layers/streetLevel/index.test.mjs

@@ -482,3 +482,17 @@ test('switching a provider on while the layer is off activates nothing (M01)', a
   assert.equal(provider.calls.activate, 1);
   assert.equal(viewer.credits.length, 1);
 });
+
+test('an empty provider composition enables and tears down without acquiring imagery', async (t) => {
+  const { layer, viewer } = await enabledLayer(t, []);
+  assert.deepEqual(layer.providerIds, []);
+  assert.deepEqual(layer.getUIState().providers, []);
+  assert.equal(layer.getUIState().keyRequired, false);
+  assert.match(
+    layer.getUIState().coverage.hint,
+    /No street-level imagery providers/,
+  );
+  layer.disable();
+  layer.enable(viewer);
+  assert.deepEqual(layer.getUIState().providers, []);
+});

src/layers/streetLevel/registry.js

@@ -86,8 +86,8 @@ const ID_GRAMMAR = /^[a-z][a-z0-9-]*$/;
  * @returns {ReadonlyArray<StreetLevelProvider>}
  */
 export function validateProviders(providers) {
-  if (!Array.isArray(providers) || providers.length === 0)
-    throw new TypeError('Street Level needs at least one imagery provider');
+  if (!Array.isArray(providers))
+    throw new TypeError('Street Level providers must be an array');
   const ids = new Set();
   const prefixes = [];
   for (const provider of providers) {

src/layers/streetLevel/registry.test.mjs

@@ -26,9 +26,14 @@ test('a valid list is frozen in registration order', () => {
   );
 });
 
-test('an empty list, duplicate ids and malformed ids are rejected', () => {
-  assert.throws(() => validateProviders([]), /at least one/);
-  assert.throws(() => validateProviders(null), /at least one/);
+test('an empty provider list is valid and has no key requirement', () => {
+  assert.deepEqual(validateProviders([]), []);
+  assert.ok(Object.isFrozen(validateProviders([])));
+  assert.equal(requiresKeyIdFor([]), null);
+});
+
+test('non-arrays, duplicate ids and malformed ids are rejected', () => {
+  assert.throws(() => validateProviders(null), /array/);
   assert.throws(() => validateProviders([def(), def()]), /Duplicate/);
   assert.throws(() => validateProviders([def({ id: 'Bad Id' })]), /slug/);
 });

src/layers/streetLevel/uiState.js

@@ -27,7 +27,10 @@ export function summarizeCoverage(providers) {
   return {
     count: active.reduce((sum, p) => sum + (p.count || 0), 0),
     loading: active.some((p) => p.loading),
-    hint: active.find((p) => p.hint)?.hint || '',
+    hint:
+      providers.length === 0
+        ? 'No street-level imagery providers configured'
+        : active.find((p) => p.hint)?.hint || '',
     error: active.find((p) => p.error)?.error || null,
     keyRequired:
       active.length > 0 && active.every((p) => p.keyRequired === true),