fix(openai oauth): drop item_reference + stored-item id refs on OAuth path

[yetone]

Problem

Every continuation request (i.e. any /v1/responses request with function_call_output items, or item_reference items, in input) routed through the OAuth path is currently guaranteed to fail with HTTP 502, masking an upstream HTTP 404:

OpenAI upstream error 404 (account=N platform=openai type=oauth):
{
  "error": {
    "message": "Item with id 'fc_<...>' not found. Items are not persisted when `store` is set to false. Try again with `store` set to true, or remove this item from your input.",
    "type": "invalid_request_error",
    "param": "input"
  }
}

In production this manifests as a 100% failure rate for any client that uses tool-calling continuation through OAuth — no successful turn after the first one is ever possible.

Root cause

The OAuth path forces store=false on the upstream request — both in applyCodexOAuthTransform (`openai_codex_transform.go` around line 138) and again as a belt-and-braces in `normalizeOpenAIPassthroughOAuthBody` (`openai_gateway_service.go` around line 5828). With store=false, ChatGPT's internal Responses backend never persists any item, so any `item_reference` (and any other item carrying an `id` from a prior turn) is guaranteed to 404.

How we got into this state:

• 70eaa45 ('fix(网关): 修复工具续链校验与存储策略') tried to fix tool continuation by (a) forcing `store=true` on continuation turns and (b) preserving `item_reference` items + per-item `id` fields in the filtered input. The two halves are load-bearing for each other: preserve-references only makes sense if store=true means upstream actually has those items.
• 3663951 ('fix(网关): OAuth 请求强制 store=false') reverted half (a) the next day after upstream returned 'Store must be set to false', restoring the unconditional `store=false` block. It did not revert half (b) — `filterCodexInputWithOptions` was left with `PreserveReferences: needsToolContinuation`.

So every continuation request now goes upstream with `store=false` plus preserved `item_reference` items / `id` fields → guaranteed 404 → 502 to the client.

Fix

Pass `PreserveReferences: false` unconditionally on the OAuth path. The function_call + matching function_call_output items inlined in the same request body already carry the full continuation context — that's the standard Responses-API continuation shape and upstream accepts it without any stored-item references.

Drop the now-vestigial `needsToolContinuation` local (it was only read by the line we're changing).

Update the three tests that had been written to lock in 70eaa45's intermediate behavior (`TestApplyCodexOAuthTransform_ToolContinuationPreservesInput`, `...PreservesNativeMessageAndReasoningIDs`, `...NormalizesToolReferenceIDsOnly`) so they assert the corrected behavior:

• `item_reference` items dropped on the OAuth path
• stored-item `id` fields stripped from surviving items
• `function_call_output` items kept; `call_id` normalized `call_` → `fc_` (unchanged)

Verification (production)

Deployed this patch on top of `cumora-prod` in our fork and rolled it to GKE. Sampled `/v1/responses` status codes on the OAuth path before and after a probe wake of 4 agent pods:

Status	Before fix	After fix (2 min sample)
200 OK	0	18
502 (this bug — 'Items are not persisted when store is set to false')	high	0
502 (unrelated 'No tool call found …' — separate client-side issue)	masked	6

The original failure mode is fully eliminated. The remaining 502s carry a different upstream message and trace to a separate client-side bug unrelated to this PR.

Test plan

• `go build ./...` succeeds
• `go vet` clean
• Three updated tests pass and now express the correct contract (item_reference dropped, stored-item id stripped, function_call_output preserved with call_id normalized)
• Verified end-to-end against a live ChatGPT OAuth account through `normalizeOpenAIPassthroughOAuthBody` → `/v1/responses` upstream