[codex-analytics] Analytics Capture to File in Debug Builds

[jameswt-oai]

This PR

The original combined remote plugin analytics PR #26281 mixed reusable analytics test infrastructure, two manual smoke workflows, a metadata refactor, and the final identity behavior. This PR isolates the generic capture mechanism so it can be reviewed and landed before any plugin-specific behavior.

• Add a debug-only analytics destination that writes final request payloads as JSONL.
• Suppress HTTP delivery whenever capture mode is selected, including after capture write failures.
• Keep release behavior unchanged even when the capture environment variable is present.
• Keep the mechanism generic; this PR contains no plugin-specific behavior.

Set CODEX_ANALYTICS_EVENTS_CAPTURE_FILE=/path/events.jsonl when running a debug Codex binary to inspect the exact batched payload that would otherwise be sent to the analytics endpoint.

Testing

• just test -p codex-analytics (76 passed)
• just test --release -p codex-analytics (73 passed)
• CI is green across the required platform matrix.

Split Overview

main
├── #27093  Debug analytics capture                 ← you are here
│   └── #27099  Non-mutating plugin smoke
│       └── #27100  Remote install/uninstall smoke
└── #27102  Plugin telemetry metadata refactor

After #27093, #27099, #27100, and #27102 merge:
└── Final PR: add remote_plugin_id to plugin analytics

Review order and dependencies:

1. #27093 Add debug-only analytics event capture (this PR, based on main)
2. #27099 Add a plugin analytics smoke workflow (stacked on [codex-analytics] Analytics Capture to File in Debug Builds #27093)
3. #27100 Add a remote plugin analytics mutation smoke workflow (stacked on [codex-app-server-test-client & codex-app-server] Plugin Usage Analytics Smoke Test #27099)
4. #27102 Centralize plugin telemetry metadata construction (independent, based on main)
5. Final remote-ID behavior PR (created after PRs 1-4 merge)

The original #26281 remains open as the green aggregate reference until the final PR is published.