GitLab

- staticcheck QF1001: apply De Morgan's law to the OAuth-mimic header
  passthrough guard (`!(a && b)` → `a != ... || !b`).
- unused: drop `isClaudeCodeRequest`, which became dead after PR #1914
  switched both `/v1/messages` and `/count_tokens` paths to unconditional
  `account.IsOAuth()` mimicry. The lowercase helper `isClaudeCodeClient`
  is kept (still referenced by `TestIsClaudeCodeClient`).