Add durable proxy acceptance fixtures

[IlyaasK]

Summary

• add durable proxy create/delete calls to the provider-local Kernel client
• add guarded, project-scoped proxy cleanup for acceptance fixtures
• verify mutation scope, disabled retries, cleanup deadlines, 404 handling, and error reporting
• keep the SDK proxy health-check operation outside the provider client surface

Why this is v0 work

The kernel_proxy data source needs a real managed datacenter proxy fixture for live acceptance coverage. These helpers create and clean up that durable fixture without adding a Terraform proxy resource or runtime proxy operations.

Tests

• go test -short -timeout=2m ./...
• go test -race -short -timeout=2m ./internal/kernelclient ./internal/acctest
• go vet ./...
• go mod verify
• go mod tidy -diff
• terraform fmt -check -recursive examples
• bash scripts/check-markdown-links.sh
• bash scripts/check-docs.sh
• git diff --check
• gofmt -l cmd internal

Acceptance status

No live API test is added in this fixture-only PR. The stacked proxy data-source acceptance PR creates the managed datacenter proxy and exercises cleanup.

Intentionally unsupported

• no Terraform proxy resource
• no custom proxy credential management
• no proxy health checks
• no browser/session runtime operations

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Note

Low Risk
Changes are limited to test infrastructure and thin API client wrappers; no provider runtime or auth paths are modified.

Overview
Adds project-scoped proxy create/delete on the provider kernelclient (mutations use disabled SDK retries, same as other resources) and CleanupProxy acceptance helpers with unit tests for env gates, default vs explicit project, 404-as-clean, deadlines, and delete errors. A reflect test documents that CheckProxy is intentionally not on the client surface.

Also wires live kernel_profile data source acceptance (API-created fixture, cleanup + post-cleanup verification) and extends the acceptance workflow matrix to run ./internal/datasources/profile with KERNEL_PROJECT_ID required.

No Terraform proxy resource or proxy data-source acceptance in this diff; proxy helpers are fixture-only for a follow-on PR.

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