fix(conformance): unwrap formae.Value wrappers in CreateUnmanagedResource

[naxty]

Summary

The discovery harness's CreateAllUnmanagedResources forwards the evaluated PKL JSON to the plugin verbatim. The agent's apply path runs resolver.ConvertToPluginFormat on properties first, which both resolves resolvable references and collapses formae.Value wrappers ({ "\$value": ..., "\$strategy": ..., "\$visibility": ... }) down to their scalar payload. The harness only does the resolvable half, so a field declared in PKL as

field = new formae.Value { value = "x"; strategy = "SetOnce" }

reaches the plugin as the wrapper object and the cloud API rejects it when the field is typed against the inner scalar. Hit on OVH:

[CreateOOB] failed to create unmanaged resource: create plugin-sdk-test-kubeiprestriction-cluster failed:
[version] Given data is not valid for type cloud.kube.VersionEnum (code: InvalidRequest)

…sent the moment kube testdata was relaxed to version: (String|formae.Value)? for SetOnce semantics.

Change

• New flattenFormaeValuesInProperties / flattenFormaeValueWalk helpers that recursively replace any object carrying \$value with that key's payload, while leaving \$res: true resolvable markers intact (those are owned by resolveResolvablesInProperties and run before this).
• Wire it into CreateAllUnmanagedResources immediately after the resolvable pass — same point at which the apply path's resolver.ConvertToPluginFormat would have done both steps in one call.
• Unit tests for the walker covering scalar wrappers, nested wrappers, wrappers carrying object payloads, \$res preservation, and untouched plain JSON.

After this lands, plugins authored against the apply-path contract no longer have to re-implement Value unwrapping in every Create. Will pair with a downstream consumer bump in formae-plugin-ovh.

Test plan

• go test -run TestFlattenFormaeValueWalk ./pkg/plugin-conformance-tests passes locally
• Conformance run against formae-plugin-ovh#40 with this version of the framework — the kube-cluster [CreateOOB] VersionEnum failure should be gone