test(tfacc): wire RDS DB + cluster parameter groups + fix round-trip fidelity

[vieiralucas]

Summary

Second batch of the RDS acceptance-tree expansion. Un-defers RDS parameter groups, which were explicitly held out of tfacc with a note about a "Modify-then-Describe round-trip [that] returns empty parameters/groups." This roots out that discrepancy and the related ones the upstream suite surfaces.

New rds-param-groups shard running _basic for:

• TestAccRDSParameterGroup
• TestAccRDSClusterParameterGroup

Fidelity fixes:

• ApplyMethod round-trip. DB and cluster parameters now preserve and return each parameter's ApplyMethod (immediate / pending-reboot). The Terraform provider defaults apply_method = "immediate" and produced a perpetual plan diff because DescribeDB(Cluster)Parameters omitted the field. Stored per-parameter via a backward-compatible #[serde(default)] map (DB groups) and a sibling JSON map (cluster groups).
• Reset actually resets. ResetDBParameterGroup / ResetDBClusterParameterGroup were no-ops. They now drop the named parameters — or all of them with ResetAllParameters=true — from the user-set map, so a parameter removed from config flips its source back to engine-default. Previously the "parameter no longer user defined" check failed after a parameter was removed.
• Cluster describe wire shape. DescribeDBClusterParameterGroups wrapped elements in the generic <member> tag, which made the AWS SDK unmarshal an empty list ("empty result" on read). It now uses the named <DBClusterParameterGroup> member tag (matching the working DB-subnet/DB-parameter-group paths), honors the DBClusterParameterGroupName filter, returns DBParameterGroupNotFound for an unknown group, and includes <Description>.

The stale deferral comment on the rds Service entry is updated to reflect that parameter groups are now wired (option groups remain deferred; DB instances/clusters still need Docker).

Test plan

• Both families pass locally against a fresh fakecloud server, and end-to-end through the harness shard (cargo test -p fakecloud-tfacc --test acc rds_param_groups_acceptance — 1 passed, 127s):
  • TestAccRDSParameterGroup_basic PASS (was: perpetual diff, then reset check)
  • TestAccRDSClusterParameterGroup_basic PASS (was: empty-result on read)
• cargo test -p fakecloud-rds — 222 passed
• cargo clippy -p fakecloud-rds -p fakecloud-tfacc --all-targets -- -D warnings clean; cargo fmt applied.

Surface check

• No new public API / introspection endpoint — these are existing RDS query-protocol response fields, so no first-party SDK change.
• DbParameterGroup gained a #[serde(default)] parameter_apply_methods map; cluster groups store apply methods in a sibling JSON key — both back-compatible with older persisted snapshots. CFN provisioner ctor updated for the new field.
• tfacc service list in docs/about/conformance.md unchanged: rds already listed; this deepens resource coverage within it.

Follow-on batches

RDS option groups (separate failure), event subscriptions and global clusters (provider panic on read — need their own investigation), then the Lambda tree.

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Summary by cubic

Enables RDS DB and cluster parameter groups in fakecloud-tfacc and fixes the Modify/Describe round-trip so Terraform no longer drifts. ApplyMethod now round-trips, resets work, and cluster group listing matches AWS.

• New Features

  • Add rds-param-groups shard to run TestAccRDSParameterGroup_basic and TestAccRDSClusterParameterGroup_basic.

• Bug Fixes

  • Preserve ApplyMethod for DB and cluster parameters; DescribeDB(Cluster)Parameters now includes it.
  • Make ResetDB(Cluster)ParameterGroup clear named or all parameters (and apply methods) to restore engine-default.
  • Fix DescribeDBClusterParameterGroups to use the <DBClusterParameterGroup> member tag, honor name filtering, include <Description>, and return DBParameterGroupNotFound for unknown groups.

^{Written for commit 300f788. Summary will update on new commits.}