fix(datareading): improve DataReading JSON parsing and error handling

[wallrj-cyberark]

Some improvements to the UnmarshalJSON function (which was introduced in #707):

• Refactor UnmarshalJSON to use prioritized type decoding for Data field
• Add strict unmarshalling helper to disallow unknown fields
• Return errors for empty or unknown data types in Data field
• Add unit tests for valid and invalid JSON scenarios

Testing

The "echo server" test was using a arbitrary data type, which is no longer allowed, so I switched it to DataReading. Here's the output of the updated test:

$ go test ./pkg/echo/... -v -count 1
=== RUN   TestEchoServerRequestResponse
-- POST /api/v1/datareadings -> created 201
received 1 readings:
0:
ClusterID: test_suite_cluster
Data gatherer: dummy
Timestamp: 2025-09-05T06:15:33+01:00
SchemaVersion: 2.0.0
Data: &{ClusterID: ServerVersion:v1.20.0}
-----
-- error 400 -> invalid method. Expected POST, received GET
--- PASS: TestEchoServerRequestResponse (0.00s)
PASS
ok      github.com/jetstack/preflight/pkg/echo  0.010s

The machinehub outputmode test still works:

go test ./cmd/... -v -count 1 -run '.*/machinehub'
=== RUN   TestOutputModes
=== RUN   TestOutputModes/machinehub
    agent_test.go:71: Running child process /tmp/go-build2356754855/b001/cmd.test -test.run=^TestOutputModes/machinehub$
    agent_test.go:80: STDOUT
        I0905 06:19:25.669617   59271 run.go:58] "Starting" logger="Run" version="development" commit=""
        I0905 06:19:25.669859   59271 config.go:448] "Output mode selected" logger="Run" mode="MachineHub" reason="--machine-hub was specified"
        I0905 06:19:25.669897   59271 run.go:116] "Healthz endpoints enabled" logger="Run.APIServer" addr=":8081" path="/healthz"
        I0905 06:19:25.669919   59271 run.go:120] "Readyz endpoints enabled" logger="Run.APIServer" addr=":8081" path="/readyz"
        I0905 06:19:25.669961   59271 run.go:269] "Pod event recorder disabled" logger="Run" reason="The agent does not appear to be running in a Kubernetes cluster." detail="When running in a Kubernetes cluster the following environment variables must be set: POD_NAME, POD_NODE, POD_UID, POD_NAMESPACE"
        I0905 06:19:25.669979   59271 run.go:308] "Reading data from local file" logger="Run.gatherAndOutputData" inputPath="/home/richard/projects/jetstack/jetstack-secure/examples/machinehub/input.json"
        I0905 06:19:25.670377   59271 run.go:432] "Starting" logger="Run.APIServer.ListenAndServe" addr=":8081"
        I0905 06:19:26.008565   59271 round_trippers.go:632] "Response" logger="Run.gatherAndOutputData.postData" verb="GET" url="https://platform-discovery.integration-cyberark.cloud/api/public/tenant-discovery?bySubdomain=tlskp-test" status="200 OK" milliseconds=337
        I0905 06:19:26.559188   59271 round_trippers.go:632] "Response" logger="Run.gatherAndOutputData.postData" verb="POST" url="https://anb5751.id.integration-cyberark.cloud/Security/StartAuthentication" status="200 OK" milliseconds=548
        I0905 06:19:26.559776   59271 identity.go:292] "made successful request to StartAuthentication" logger="Run.gatherAndOutputData.postData" source="Identity.doStartAuthentication" summary="NewPackage"
        I0905 06:19:27.173640   59271 round_trippers.go:632] "Response" logger="Run.gatherAndOutputData.postData" verb="POST" url="https://anb5751.id.integration-cyberark.cloud/Security/AdvanceAuthentication" status="200 OK" milliseconds=613
        I0905 06:19:27.174260   59271 identity.go:402] "successfully completed AdvanceAuthentication request to CyberArk Identity; login complete" logger="Run.gatherAndOutputData.postData" username=""
        I0905 06:19:32.908654   59271 round_trippers.go:632] "Response" logger="Run.gatherAndOutputData.postData" verb="POST" url="https://tlskp-test.inventory.integration-cyberark.cloud/api/ingestions/kubernetes/snapshot-links" status="200 OK" milliseconds=5734
        I0905 06:19:33.378071   59271 round_trippers.go:632] "Response" logger="Run.gatherAndOutputData.postData" verb="PUT" url="" status="200 OK" milliseconds=468
        I0905 06:19:33.378305   59271 run.go:417] "Data sent successfully" logger="Run.gatherAndOutputData.postData"        
        I0905 06:19:33.378351   59271 run.go:446] "Shutting down" logger="Run.APIServer.ListenAndServe" addr=":8081"        
        I0905 06:19:33.378646   59271 run.go:461] "Shutdown complete" logger="Run.APIServer.ListenAndServe" addr=":8081"
        
    agent_test.go:81: STDERR
        
--- PASS: TestOutputModes (7.80s)
    --- PASS: TestOutputModes/machinehub (7.80s)
PASS
ok      github.com/jetstack/preflight/cmd       7.864s