[WIP] Fix issues with error checks

[jakubno]

Note

Standardizes error handling across API, orchestrator, and proxy by replacing type assertions with errors.As/Is and introducing sentinel errors; updates related tests.

• Errors/Handling:
  • Replace type-based checks with errors.As/errors.Is in packages/api/internal/handlers/*, orchestrator/*, and sandbox block code.
  • Introduce/use sentinel errors instead of custom types:
    • Auth: ErrNoAuthHeader now a sentinel (errors.New).
    • Proxy: replace InvalidHostError/InvalidSandboxPortError types with sentinel vars and use errors.Is in handler.go and host.go.
• API:
  • sandbox_kill.go: handle db.EnvNotFoundError via errors.As.
  • template_build_status.go: handle db.TemplateBuildNotFoundError via errors.As.
• Orchestrator:
  • pause_instance.go: detect PauseQueueExhaustedError via errors.As.
  • NBD pool: ReleaseDeviceWithRetry retries on DeviceInUseError via errors.As.
  • Block cache: chunk.go and overlay.go use errors.As for BytesNotAvailableError.
• Tests:
  • Update memory store reservation tests to use require.ErrorAs for LimitExceededError and AlreadyBeingStartedError.

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[jakubno]

Ideally we add https://github.com/fillmore-labs/errortype, but it isn't part of golangci-lint, so we would have to do custom setup.

I created fillmore-labs/errortype#5, let's see

[cursor]

Bug: Redundant Error Variables in Function

The code declares two distinct variables of type PauseQueueExhaustedError (pauseQueueErrPauseExhausted and pauseQueueErr) for errors.As checks within the same function. This introduces redundant variable declarations and could lead to confusion.