Add durable extension acceptance fixtures

[IlyaasK]

Summary

• add durable extension upload/delete calls to the provider-local Kernel client
• add guarded, project-scoped extension cleanup for acceptance fixtures
• verify mutation scope, disabled retries, cleanup deadlines, 404 handling, and error reporting
• keep extension download and Chrome Store operations outside the provider client

Why this is v0 work

The existing kernel_extension data source needs a real uploaded fixture for live acceptance coverage. These helpers create and clean up that durable fixture without adding the parked Terraform extension resource.

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 extension data-source acceptance PR uploads the fixture and exercises cleanup.

Intentionally unsupported

• no Terraform extension resource
• no extension download or Chrome Store operations
• no browser/session runtime operations

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Note

Low Risk
Changes are limited to acceptance helpers and a narrow client surface, following established proxy/profile cleanup patterns with no provider resource or auth changes.

Overview
Adds project-scoped extension upload and delete on the provider kernelclient, using the same mutation rules as other resources (explicit project header, no SDK retries on writes).

Adds CleanupExtension in acctest so acceptance tests can tear down uploaded extension fixtures: gated on acceptance env/API key, default project resolution, timed delete context, 404 treated as clean, and other delete errors surfaced.

Tests cover cleanup behavior (including the new extension_test.go) and fold upload/delete into the existing mutation-scope table; a reflect test asserts download / Chrome Store extension APIs stay off Clients.

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