fix(codex-transform): preserve underscore when rewriting call_* tool-call ids

`fixCallIDPrefix` builds malformed ids when the input has the standard
OpenAI `call_<nanoid>` prefix:

  input:  call_YYen1qxDejd2myJwcTCf7Nyp
  output: fcYYen1qxDejd2myJwcTCf7Nyp   ← no underscore between 'fc' and the nanoid

ChatGPT's codex backend then rejects the replayed item with:

  400 Invalid 'input[N].id': 'fcYYen1qxDejd2myJwcTCf7Nyp'.
       Expected an ID that contains letters, numbers, underscores, or
       dashes, but this value contained additional characters.

Sub2api wraps that into 502 to the client. Clients using the OpenAI SDK
on the OAuth/codex path see every multi-hop turn (after the first tool
call) fail because the item_reference rewritten this way gets sent on
every subsequent hop.

The other two branches of the same function correctly emit `fc_`
(line 1029: pass-through when already `fc*`; line 1035 fallback:
`fc_" + id`). Only the `call_` → `fc_` rewrite was missing the
underscore — looks like a copy-paste slip during the original commit.

Fix: change `"fc"` to `"fc_"` on the call_ branch. One character.

Repro:
  client (OpenAI SDK) sends a function_call_output whose call_id is
  `call_<nanoid>` (default OpenAI format). The sub2api request body
  also contains an item_reference whose id mirrors the call_id (also
  `call_<nanoid>`). On the codex OAuth path, this rewrite fires for
  the item_reference's id, producing the malformed value.

Affects: `platform=openai type=oauth` accounts whose clients use the
official OpenAI SDK / Responses API conventions (id prefix `call_`).
API-key accounts and bridge-mode requests are untouched.

backend/internal/service/openai_codex_transform.go

@@ -1030,7 +1030,7 @@ func filterCodexInputWithOptions(input []any, opts codexInputFilterOptions) []an
 				return id
 			}
 			if strings.HasPrefix(id, "call_") {
-				return "fc" + strings.TrimPrefix(id, "call_")
+				return "fc_" + strings.TrimPrefix(id, "call_")
 			}
 			return "fc_" + id
 		}