apigatewayv2: GetApiMappings and DeleteApiMapping are unimplemented

[scttfrdmn]

aws apigatewayv2 get-api-mappings --domain-name <d> fails:

An error occurred (NotFoundException) when calling the GetApiMappings operation:
APIGatewayV2Plugin: unknown operation for GET /v2/domainnames/api.example.com/apimappings

parseAPIGatewayV2Operation (emulator/apigatewayv2_plugin.go:251-256) routes only
POST /v2/domainnames/{name}/apimappings → CreateApiMapping. GET on the same
path, and the /{apiMappingId} sub-path entirely, fall through to the default
branch's NotFoundException.

Found while verifying #529 end to end over the wire; it is a missing operation,
not a wrong response shape, so it was deliberately left out of that release rather
than folded into a wire-format change.

Missing today

Operation	Method + path
GetApiMappings	GET /v2/domainnames/{domainName}/apimappings
GetApiMapping	GET /v2/domainnames/{domainName}/apimappings/{apiMappingId}
UpdateApiMapping	PATCH /v2/domainnames/{domainName}/apimappings/{apiMappingId}
DeleteApiMapping	DELETE /v2/domainnames/{domainName}/apimappings/{apiMappingId}

CreateApiMapping already stores state under apigwv2APIMappingKey(account, region, domain, mappingID), but nothing indexes the mapping IDs for a domain, so a list
handler needs an ID-set key alongside it — the pattern the other v2 collections
already use (apigwv2RouteIDsKey and friends).

Already in place

The response shape work is done: v2APIMappingOut / v2APIMappingWire
(emulator/apigatewayv2_wire.go) and the apigwV2ItemsOut[T] envelope both exist
and are tested, so a list handler is a routing + state-indexing change rather than a
shape one. Verified over the wire that create-api-mapping --query ApiMappingId
parses correctly.

Acceptance criteria

• An ID-set state key for a domain's mappings, written by CreateApiMapping and
  pruned by DeleteApiMapping.
• All four operations routed in parseAPIGatewayV2Operation and dispatched.
• GetApiMappings returns apigwV2ItemsOut[v2APIMappingOut] — the items
  envelope, so an SDK parses it.
• GetApiMapping / DeleteApiMapping on an unknown id return
  NotFoundException, matching what the other v2 getters do.
• Tests assert on literal wire keys (map[string]any), per API Gateway: state structs are marshalled straight to the wire, so responses are PascalCase and carry substrate's internal AccountID/Region #529's lesson that a
  Go round-trip agrees with itself whatever the tags say.
• docs/services.md operation table regenerated.