fix: fully revert agent identity runtime wiring (#18757)

## Summary

This PR fully reverts the previously merged Agent Identity runtime
integration from the old stack:
https://github.com/openai/codex/pull/17387/changes

It removes the Codex-side task lifecycle wiring, rollout/session
persistence, feature flag plumbing, lazy `auth.json` mutation,
background task auth paths, and request callsite changes introduced by
that stack.

This leaves the repo in a clean pre-AgentIdentity integration state so
the follow-up PRs can reintroduce the pieces in smaller reviewable
layers.

## Stack

1. This PR: full revert
2. https://github.com/openai/codex/pull/18871: move Agent Identity
business logic into a crate
3. https://github.com/openai/codex/pull/18785: add explicit
AgentIdentity auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate auth callsites
through AuthProvider

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/core/tests/suite/otel.rs

@@ -228,7 +228,6 @@ async fn process_sse_records_failed_event_when_stream_closes_without_completed()
                 .features
                 .disable(Feature::GhostCommit)
                 .expect("test config should allow feature update");
-            config.model_provider.stream_max_retries = Some(0);
         })
         .build(&server)
         .await
@@ -407,7 +406,6 @@ async fn process_sse_failed_event_logs_missing_error() {
                 .features
                 .disable(Feature::GhostCommit)
                 .expect("test config should allow feature update");
-            config.model_provider.stream_max_retries = Some(0);
         })
         .build(&server)
         .await