Remove free-form analytics error subreason

[shiny-code-bot]

Why

Invalid-request response text can contain provider- or user-derived content. Copying that text into the free-form codex_error_subreason telemetry field creates unnecessary privacy risk when the existing structured error category already provides the diagnostic signal we need.

What changed

• Remove codex_error_subreason from analytics fact construction, event serialization, and reducer plumbing.
• Remove the 512-byte invalid-request text truncation helper and its unused constant.
• Update analytics fixtures and assertions so invalid-request text is absent while structured error reporting remains intact.

Validation

• cargo test -p codex-analytics — 71 tests passed.
• Repository search confirms there is no remaining runtime emission path for codex_error_subreason.

Closes #429