fix(disposablerequest): prevent stuck external-create-pending

[davidferlay]

Description of your changes

DisposableRequest gets stuck in external-create-pending after recreation or failed Create calls.

The crossplane-runtime creation guard hard-stops reconciliation because the controller does not declare its external name as deterministic.

Fix:

• Add managed.WithDeterministicExternalName(true) to both cluster-scoped and namespaced DisposableRequest reconcilers
• This tells the runtime it is safe to proceed past ambiguous creation state, since the external name is always the Kubernetes object name and the HTTP call is stateless

Fixes #176

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

• Full test suite passes (23 packages)
• Added regression test ResourceNotSyncedWithCreatePending: verifies Observe returns ResourceExists: false when Synced=false even with external-create-pending annotation set
• Validated manually on GKE 1.31 with provider-http v1.0.14 and crossplane-runtime v2.0.0