Remove ErrAbortedRPC

[ChrisSchinnerl]

With AddFailedRPC being exposed now for the Go SDK to use it the pattern we used with ErrAbortedRPC becomes more confusing.

To clean this up, this PR removes ErrAbortedRPC and updates the client to only count failed RPCs if they were not caused by contexts. If they were, the client leaves it up to the caller to know what their intentions were.

[claude]

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[Copilot]

Pull request overview

Removes the ErrAbortedRPC sentinel and the corresponding "decorate err with context cause" plumbing in the v2 client. The client now decides whether to record a failed RPC purely from ctx.Err() (context-cancelled/deadline RPCs are no longer counted as host failures), and callers that still want those treated as failures call the now-public, error-less AddFailedRPC directly. Call sites that previously relied on ErrAbortedRPC to suppress failure accounting are switched to plain WithTimeout/WithCancel, and slabs/downloads.go now manually demotes initial-shard hosts that were cancelled.

Changes:

• Remove ErrAbortedRPC and simplify AddFailedRPC to take just a host key.
• Replace WithCancelCause(..., ErrAbortedRPC) / WithTimeoutCause(..., ErrAbortedRPC) with plain WithCancel / WithTimeout across slabs and contracts code.
• Add isFailedRPC helper in the client and explicit AddFailedRPC calls for slow initial shards in downloadShards.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
client/v2/client.go	Adds isFailedRPC; switches deferred accounting to context-based check; updates exported AddFailedRPC to delegate to provider.
client/v2/hostqueue.go	Removes ErrAbortedRPC and recordFailure; collapses logic into AddFailedRPC(hostKey).
client/v2/hostqueue_test.go	Updates calls to new AddFailedRPC signature.
client/v2/rpc_test.go	Removes the TestRPCFnErrorDecoration test and now-unused mocks.
contracts/maintenance.go	Switches WithTimeoutCause(..., ErrAbortedRPC) to WithTimeout.
contracts/pinning.go	Same conversion for pin timeout.
contracts/pruning.go	Same conversion for prune timeout.
slabs/dataintegrity.go	Same conversion for integrity-check timeout.
slabs/downloads.go	Replaces dual cancel-with-cause contexts with a single WithCancel; manually records a failed RPC for cancelled initial-shard hosts.
slabs/manager.go	Adds AddFailedRPC to the HostClient interface.
slabs/manager_test.go	Adds no-op mock implementation of AddFailedRPC.

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