Tool output encode failure marks unrelated concurrent tool calls as failed

Description

When several tool calls run concurrently in one turn and one of them fails to encode its output for storage (ToolOutputStore.StorageError with operation: "encode"), the runner marks every other still-running tool call in that turn as failed too, even if they would have succeeded on their own.

This happens in packages/core/src/session/runner/llm.ts around the awaitToolFibers race: once it reports a failure, failUnsettledTools sweeps everything not yet settled with "Tool execution failed", regardless of whether the other calls' own execution was fine.

An encode failure is specific to the one tool whose output couldn't be serialized — it doesn't mean anything is wrong with the others. A write failure is more likely a shared storage problem and probably should keep failing fast.

This can also cause the model to unnecessarily retry a tool call it believes failed on the next turn, wasting an extra provider round-trip and potentially re-executing a side effect that already succeeded (e.g. writing the same file twice).

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