fix(opencode): keep each request on its own member list

[maxmad64bis]

Stacked on #14149. This branch is cut from #14149 and will be rebased after it merges.

Summary

• Before: two concurrent requests on one shared executor could replace each other's member list mid-run → After: each request walks its own member list built from its own credentials.
• The per-request list reuses the parent request-keyed state pattern (state indexed by request body identity, never on the shared instance); the shared pick index is kept and clamped to the local list, and per-member health is kept in a named store with write-back so selection order for a single request is unchanged.

Related Issues

• Related to fix(opencode): keep the target format and client session per request, not on the shared executor #14149

Validation

• Change type: provider
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/opencode-accounts-per-request.test.ts — overlap isolation for two interleaved requests with different credentials, shared pick-index continuity across sequential requests, cooldown survival across re-sync, cold default member, and fleet to direct to fleet index reset.
• New test file passes 5/5; neighbouring executor suites pass with 0 failures.

Coverage Notes

• The changed executor paths in open-sse/executors/opencode.ts and the new open-sse/executors/opencodeAccountScope.ts are covered by the new overlap test (isolation, index continuity, health write-back) plus the existing neighbouring executor tests that guard single-request selection order.
• No coverage drop is expected in the touched files; any move will be recovered with an added case in the same new test file.

Reviewer Notes

• Delta vs the parent base: 1 commit touching open-sse/executors/opencode.ts, new open-sse/executors/opencodeAccountScope.ts, and one new test file; rebase needed once fix(opencode): keep the target format and client session per request, not on the shared executor #14149 merges.
• Behaviour to check: shared pick index kept (reset only on the single direct-member path), per-member health preserved across re-sync, entry reuse when the same request body re-enters, release in execute.
• Manual check: two concurrent requests on two connections of the same alias each dispatch on their own list, with masked identifiers only in logs.

[maxmad64bis]

Superseded by #14353 — rebuilt as a fresh PR on the current release base (park/pacing reconciled, no parent dependency).