agent: treat unrecognized codex features output as a failed probe

knownFeatures only failed on an empty parse, so output in a format it did not expect (a header, a table, JSON) parsed as a non-empty set naming none of the real features. GenerateText then filtered every --disable out and ran codex exec with shell_tool, unified_exec, and code_mode_host enabled. Require shell_tool in the probe output; without it the full list is passed.