BOLT-1263: resolveEnv returns 400 (not 502) for unprovisioned explicit body.env

Cursor on PR #12160 flagged that resolveEnv only mapped unconfigured-env to 400 on the implicit branch. Explicit body.env=production on a staging-only deploy fell through to getItronConfig which threw and asyncHandler answered 502 instead of 400. Added up-front getItronConfig(bodyEnv) check inside resolveEnv with ItronConfigError->400 mapping; non-ItronConfigError throws still propagate to 502 via asyncHandler. PR #12247 opened. Branch artem-agent/bolt-1263-resolveenv-explicit-unconfigured. Full suite 497 passing.