test: wire FDv1 Fallback Directive into contract test service

Declare the fdv1-fallback capability and accept the new top-level
dataSystem.fdv1Fallback config object, wiring it directly to the SDK's
FDv1 Fallback Synchronizer. Drop the heuristic that previously inferred
the FDv1 fallback from the last polling synchronizer entry -- the
FDv1 Fallback Synchronizer is a distinct configuration concern from
the FDv2 Primary/Fallback chain.

Also bump the test harness pin to v3.0.0-alpha.6 so the new
fdv1-fallback test suite runs in CI.

Author: keelerm84

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
         with:
           test_service_port: 9000
           token: ${{ secrets.GITHUB_TOKEN }}
-          version: v3.0.0-alpha.4
+          version: v3.0.0-alpha.6
           enable_persistence_tests: "true"
 
   windows:

contract-tests/client_entity.py

@@ -62,7 +62,6 @@ def __init__(self, tag, config):
             sync_configs = datasystem_config.get('synchronizers')
             if sync_configs is not None:
                 sync_builders = []
-                fallback_builder = None
 
                 for sync_config in sync_configs:
                     streaming = sync_config.get('streaming')
@@ -79,14 +78,18 @@ def __init__(self, tag, config):
                         _set_optional_time(polling, "pollIntervalMs", builder.poll_interval)
                         sync_builders.append(builder)
 
-                        fallback_builder = fdv1_fallback_ds_builder()
-                        _set_optional_value(polling, "baseUri", fallback_builder.base_uri)
-                        _set_optional_time(polling, "pollIntervalMs", fallback_builder.poll_interval)
-
                 if sync_builders:
                     datasystem.synchronizers(*sync_builders)
-                if fallback_builder is not None:
-                    datasystem.fdv1_compatible_synchronizer(fallback_builder)
+
+            # The FDv1 Fallback Synchronizer is engaged only in response to a
+            # server-directed FDv1 Fallback Directive; it is configured separately
+            # from the FDv2 Primary/Fallback synchronizer chain.
+            fdv1_fallback_config = datasystem_config.get('fdv1Fallback')
+            if fdv1_fallback_config is not None:
+                fallback_builder = fdv1_fallback_ds_builder()
+                _set_optional_value(fdv1_fallback_config, "baseUri", fallback_builder.base_uri)
+                _set_optional_time(fdv1_fallback_config, "pollIntervalMs", fallback_builder.poll_interval)
+                datasystem.fdv1_compatible_synchronizer(fallback_builder)
 
             if datasystem_config.get("payloadFilter") is not None:
                 opts["payload_filter_key"] = datasystem_config["payloadFilter"]

contract-tests/service.py

@@ -84,6 +84,7 @@ def status():
             'persistent-data-store-consul',
             'flag-change-listeners',
             'flag-value-change-listeners',
+            'fdv1-fallback',
         ]
     }
     return json.dumps(body), 200, {'Content-type': 'application/json'}