IsTerminal must not kill the turn on recoverable failures

[timothyerwin]

A real bug in v0.31.0, found while setting up a test PR to verify the reviewer.

The bug

v0.31.0 added IsTerminal so a delegated worker's terminal failure ends the turn instead of coming back as a retryable tool error the model retries forever. It was written as a one-line alias:

func IsTerminal(err error) bool { return err != nil && terminalForFallback(err) }

But those two functions answer different questions.

terminalForFallback asks "should the fallback walk stop here?" Several of its cases stop the walk precisely because something else recovers them:

cause	what actually recovers it
ErrContextOverflow	compact-and-retry in runLoop
ErrStreamIncomplete	same-call transport retry
ErrLaneExhausted	raises the user's fallback-choice card

IsTerminal asks the narrower question: "does this end the turn?" Aliasing them means a worker that merely overflowed its context now kills the parent turn instead of compacting — replacing a working recovery with a hard failure.

The irony is that the failure would look like a sub-agent dying abruptly on a long task, which is close to the hang this classification was added to prevent.

The fix

IsTerminal excludes the recoverable causes explicitly, then defers to terminalForFallback for the rest. Request-shape 4xx, auth, and billing states stay terminal; overflow, stream cuts, rate limits, 5xx and transport failures do not.

Verification

The new table test fails against the shipped v0.31.0 build (context overflow and incomplete stream both report IsTerminal = true) and passes here. Full gate clean: gofmt, go vet, go test -race ./....

Why this PR exists

Opened partly as a live end-to-end check that PR review works again after the Gemini thought-signature fix chain (#8, released as v0.31.0, reviewer image rebuilt at that tag). It turned into a real fix, which is a better test than a dummy edit.

[memcode-ai]

Walkthrough

Separates IsTerminal from terminalForFallback so recoverable errors do not abort the parent agent turn. Worker failures caused by context overflow, incomplete streams, or exhausted lanes now remain recoverable rather than triggering turn-fatal errors.

• Explicitly filters out wire.ErrContextOverflow, wire.ErrStreamIncomplete, and provider.ErrLaneExhausted before delegating to terminalForFallback.
• Adds TestIsTerminalExcludesRecoverableCauses covering malformed requests, auth issues, context overflows, rate limits, transport failures, and lane exhaustion.

The change cleanly isolates turn-killing terminal errors from fallback-stopping conditions and is well-covered by targeted tests.

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No findings.