Split overloaded error codes 104001-104003

[lmars]

AIT-1259

Three of the SDK's error codes each covered several unrelated failures, so a developer could not switch on the code and react appropriately. Writing the registry entries for ably-common#353 exposed this — each entry had to say "this error takes two forms, with different fixes", which is a smell that the code itself is wrong.

The splits

Was	Failure	Now
104001	Channel subscribe/attach failed during connect() — nothing sends or receives	104001 SessionSubscriptionError
104001	Processing one inbound message threw — the session keeps working	104009 SessionMessageProcessingFailed
104002	onCancel threw — the SDK never reaches the abort, so the run is not cancelled	104002 RunCancelHandlerFailed
104002	onSteer threw — the steering message already folded in, so only the notification failed	104012 RunSteerHandlerFailed
104002	Cancel-message routing failed — not a developer-handler fault at all	104013 RunCancelRoutingFailed
104003	A run-lifecycle publish failed	104003 RunLifecycleEventPublishFailed

Each now describes one failure with one recovery, and the recovery differs in each case: retry connect() versus fix your handler; the run is dead versus the run is fine.

104003 now covers steps

publishLifecycle() wrapped the run-lifecycle publishes only. The step-lifecycle publishes — runManager.startStep() and endStep() — let the raw Ably publish error propagate, though a failed ai-step-start is the same class of failure as a failed ai-run-start.

That bracket moves to src/core/transport/lifecycle-publish.ts and both tiers now use it, so neither can drift on the error code, message shape, or cause preservation. The code is renamed to reflect the widened scope.

Naming

Members are named after their registry identifier, which keeps a run_ or session_ prefix so the AIT codes group together. These are the names that generating the constants from the registry (ably-common#357) will derive, so adopting them now avoids renaming the same members twice. The members this PR does not touch still differ from their identifiers; that migration is what settles them.

ErrorCode.CancelListenerError is removed and RunLifecycleError renamed, so this is a breaking change to the exported ErrorCode enum.

ably-common

Pinned at the merge of #353. protocol/errors.json no longer maps a code straight to a description string — entries sit under a codes envelope, each an object with an identifier, title and summary. validate-error-codes.ts indexed the top level, so against the bumped pin it would have found nothing and reported all 19 codes as missing. It now reads codes, and reports an absent envelope as a stale or uninitialised submodule rather than letting it read as 19 unregistered codes.

Testing

check:error-codes resolves all 19 codes. New unit coverage for each newly distinguished failure: the onSteer throw, the cancel-routing failure, and a failed ai-step-start / ai-step-end publish. The routing path is reachable when a run's own onError throws while reporting an onCancel failure — the second throw escapes the cancel handler and lands in the routing bracket — and the test drives it that way. publishLifecycleEvent gets its own test file.

Full suite green: 69 files, 1677 tests.

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