fix(runtime-fallback): preserve attemptCount when our own abort is the cause (closes #4006)

When runtime-fallback aborts an in-flight request to swap in a fallback
model, opencode emits session.error{isAbort:true} as a consequence. The
existing event handler treated that as a user cancellation and called
resetRetryState — wiping attemptCount. Every subsequent provider
auto-retry signal then started over at attempt:1, never reaching
max_fallback_attempts, producing an infinite retry loop firing a new
fallback every ~2 seconds.

The bug only surfaces when the configured fallback target itself
silently fails (e.g. github-copilot quota exhausted): the original
model keeps re-emitting retry signals, our handler keeps "fixing"
them, the counter never advances. Reproducible on upstream/dev HEAD
(5ffbe0e24e).

Fix:
- New `internallyAbortedSessions: Set<string>` on HookDeps tracks
  sessions whose abort we triggered ourselves.
- abortSessionRequest in auto-retry.ts adds the session to the set
  when called with one of our internal sources:
  "session.status.retry-signal", "message.updated.retry-signal",
  "session.timeout". The "session.stop" source (user-initiated) is
  intentionally NOT marked — that path must still wipe state.
- handleSessionError in event-handler.ts checks the set before the
  cancellation branch. If the session is marked, consume the flag
  (delete it so a later user-abort still gets the reset) and skip
  resetRetryState. The state's attemptCount is preserved, so the
  next iteration progresses 1→2→3→... until max_fallback_attempts.
- dispose() clears the new set alongside the other per-session maps.

Tests: 3 new event-handler integration tests cover the fix
(internal-abort preserves state, external-abort still resets,
consecutive internal-abort cycles advance attemptCount). Existing
tests pass: 7/7 on event-handler. Pre-existing 2 dispose-test flakes
on the full runtime-fallback suite were verified to exist on
upstream/dev without this patch — unrelated.

bun run build: pass. bun run typecheck: pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/hooks/runtime-fallback/event-handler.ts

@@ -138,6 +138,14 @@ export function createEventHandler(deps: HookDeps, helpers: AutoRetryHelpers) {
     const resolvedAgent = await helpers.resolveAgentForSessionFromContext(sessionID, agent)
 
     if (isAbortError(error)) {
+      // If we triggered this abort to swap in a fallback model, consume the
+      // flag and preserve state — wiping attemptCount here is what causes
+      // the infinite retry loop (issue #4006).
+      if (deps.internallyAbortedSessions.has(sessionID)) {
+        deps.internallyAbortedSessions.delete(sessionID)
+        log(`[${HOOK_NAME}] session.error matched internal abort; preserving retry state`, { sessionID, resolvedAgent })
+        return
+      }
       cancelledSessions.add(sessionID)
       resetRetryState(sessionID)
       log(`[${HOOK_NAME}] session.error matched cancellation; cleared retry state`, { sessionID, resolvedAgent })