fix(rds): persist Copy/Modify/Restore extras + activity-stream + PI fields (bug-hunt)

[vieiralucas]

Summary

A cluster of RDS Copy*/Modify*/Restore* "extras" ops returned canned/hardcoded XML but never persisted to state, so a subsequent Describe showed nothing (or a default). Their cluster-level analogs already persisted correctly a few hundred lines away in the same file. This mirrors that persistence pattern so every op round-trips through its Describe.

Behavior-only fix. No SDK/doc/operation-count surface change.

HIGH

• CopyDBSnapshot now reads the source DBSnapshot, clones it (including dump data) into state.snapshots under the target id, and renders from the persisted record. DescribeDBSnapshots --db-snapshot-identifier <target> now returns the copy instead of DBSnapshotNotFoundFault.

MEDIUM

• CopyDBParameterGroup persists the copy into state.parameter_groups.
• ModifyDBSnapshotAttribute / DescribeDBSnapshotAttributes persist the attribute set (e.g. restore add/remove values) on the snapshot and read it back; rejects a value present in both add and remove lists. ModifyDBSnapshot now applies EngineVersion/OptionGroupName.
• ModifyCurrentDBClusterCapacity applies the requested Capacity to the cluster and echoes real values (was hardcoded CurrentCapacity=4 + cluster id x); the capacity round-trips through DescribeDBClusters.
• EnableHttpEndpoint / DisableHttpEndpoint write HttpEndpointEnabled onto the cluster (via ResourceArn) the same way ModifyDBCluster does; DescribeDBClusters reflects it. Unknown resource returns the declared ResourceNotFoundFault.
• RestoreDBClusterFromS3 creates + persists a real cluster (was empty <DBCluster/>).
• ModifyDBInstance now applies MonitoringRoleArn, PerformanceInsightsKMSKeyId, PerformanceInsightsRetentionPeriod (their companions were already applied).

LOW

• ModifyDBSubnetGroup applies DBSubnetGroupDescription.
• ModifyDBShardGroup / CreateDBShardGroup persist MaxACU/MinACU/ComputeRedundancy.
• StartActivityStream / StopActivityStream / ModifyActivityStream persist an activity-stream status/mode/kms/kinesis onto the DB instance (new DbInstance::activity_stream field) or Aurora cluster named by ResourceArn, rendered by DescribeDBInstances/DescribeDBClusters (was hardcoded <ActivityStreamStatus>stopped). ModifyActivityStream/HTTP-endpoint ops now return only Smithy-declared error shapes.

State-struct change: added DbInstance::activity_stream (+ ActivityStreamConfig) and DbSnapshot::snapshot_attributes; swept all constructors repo-wide (including fakecloud-cloudformation).

Test plan

• New unit/integration round-trip tests in crates/fakecloud-rds/src/service_tests.rs and extras/tests.rs: copy-then-describe, modify-attribute-then-describe, enable-http-endpoint-then-describe, capacity echo + describe round-trip, restore-from-s3, shard-group capacity, activity-stream persist, PI/monitoring-role apply, subnet-group description.
• New E2E in crates/fakecloud-e2e/tests/rds_extras_persistence.rs (real AWS SDK, container-free): http endpoint, capacity, copy param group, restore-from-s3, activity stream on a cluster, subnet-group description. All 6 pass.
• cargo nextest run -p fakecloud-rds — 255 passing.
• cargo clippy -p fakecloud-rds --all-targets -- -D warnings clean; cargo fmt.
• Conformance check PASSED with no regressions; RDS moved from 5487/5487 baseline to 5500/5518 with every changed op at 100% (fixed an initial regression where http-endpoint/activity-stream ops returned undeclared error shapes).

---

Summary by cubic

Fixes RDS extras so Copy/Modify/Restore operations persist to state and round-trip through Describe, and makes activity stream and performance insights settings stick. Removes canned responses, returns correct error shapes, and improves parity with AWS.

• Bug Fixes

  • Snapshots: CopyDBSnapshot now clones the source into state (handles KMS/OptionGroup; no inherited share attrs); ModifyDBSnapshot applies EngineVersion/OptionGroupName; ModifyDBSnapshotAttribute persists the restore set and rejects values present in both add/remove; DescribeDBSnapshotAttributes returns saved attributes.
  • Parameter Groups: CopyDBParameterGroup persists the target group with description/family.
  • Clusters: ModifyCurrentDBClusterCapacity writes Capacity and echoes real values; Enable/DisableHttpEndpoint persist HttpEndpointEnabled by ResourceArn and return ResourceNotFoundFault for unknown ARNs; RestoreDBClusterFromS3 creates a real, describable cluster.
  • Instances: ModifyDBInstance applies MonitoringRoleArn, PerformanceInsightsKMSKeyId, and PerformanceInsightsRetentionPeriod.
  • Subnet/Shards: ModifyDBSubnetGroup updates DBSubnetGroupDescription; Create/ModifyDBShardGroup persist MaxACU/MinACU/ComputeRedundancy; RebootDBShardGroup echoes the stored group.
  • Activity Stream: Start/Stop/Modify persist stream status/mode/kms/kinesis on the targeted cluster or instance; DescribeDBClusters/DescribeDBInstances reflect it; ModifyActivityStream uses only declared error shapes.

• Migration

  • State additions: DbInstance::activity_stream and DbSnapshot::snapshot_attributes. Constructors updated across crates and tests to default activity_stream to None. No SDK or operation surface changes.

^{Written for commit b23b1f4. Summary will update on new commits.}

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