[codex] Use AgentAssertion downstream behind use_agent_identity

[adrian-openai]

Summary

This is the AgentAssertion downstream slice for feature-gated agent identity support, replacing the oversized AgentAssertion slice from PR #17807.

It isolates downstream AgentAssertion wiring on top of PR3.1 (dev/adrian/codex/agent-task-state-prewarm) without re-carrying the earlier agent registration, task registration, or task-state history.

Stack

• PR1: Add use_agent_identity feature flag #17385 - add features.use_agent_identity
• PR2: Register agent identities behind use_agent_identity #17386 - register agent identities when enabled
• PR3: Register agent tasks behind use_agent_identity #17387 - register agent tasks when enabled
• PR3.1: Persist and prewarm agent tasks per thread #17978 - persist and prewarm registered tasks per thread
• PR4: [codex] Use AgentAssertion downstream behind use_agent_identity #17980 - this PR, use AgentAssertion downstream when enabled

What Changed

• add AgentAssertion envelope generation in codex-core
• route downstream HTTP and websocket auth through AgentAssertion when an agent task is present
• add targeted client and assertion tests for the new auth path
• extend CoreAuthProvider so non-bearer authorization schemes can be passed through cleanly

Why

The original PR had drifted ancestry and showed a much larger diff than the semantic change actually required. Restacking it onto PR3.1 keeps the reviewable surface down to the downstream assertion slice.

Validation

• just fmt
• cargo test -p codex-api core_auth_provider
• cargo test -p codex-core --lib authorization_header_for_task
• cargo test -p codex-core --lib client::tests::responses_http_uses_agent_assertion_when_agent_task_is_present
• cargo test -p codex-core websocket_agent_task_bypasses_cached_bearer_prewarm

[efrazer-oai]

I had Codex go find all the places that are still hardcoded to use Bearer as discussed on Slack, here are the results:

As it stands, we have successfully migrated the shared Responses HTTP, Responses websocket, compact, and ChatGPT-backend realtime-call paths.

But there are still several other backend surfaces that build Authorization: Bearer ... directly or go through helpers that only understand bearer tokens. Those still need to migrate if we want this change to be exhaustive.

Shared backend client paths that still need the new auth shape:

• codex-rs/backend-client/src/client.rs
  • /wham/usage
  • /wham/tasks/list
  • /wham/tasks/{id}
  • /wham/tasks/{id}/turns/{turn_id}/sibling_turns
  • /wham/config/requirements
  • POST /wham/tasks

Current callers of that shared client include:

• codex-rs/cloud-requirements/src/lib.rs
• codex-rs/app-server/src/codex_message_processor.rs
• codex-rs/cloud-tasks-client/src/http.rs

Direct backend callers that still attach bearer auth themselves:

• codex-rs/codex-api/src/files.rs
  • /files
  • /files/{id}/uploaded
• codex-rs/core/src/mcp_openai_file.rs
• codex-rs/models-manager/src/manager.rs
  • /codex/models
• codex-rs/codex-mcp/src/mcp/mod.rs
  • .../wham/apps
• codex-rs/chatgpt/src/chatgpt_client.rs
  • shared GET helper used by:
    • codex-rs/chatgpt/src/get_task.rs
    • connector directory loading paths
• codex-rs/core/src/connectors.rs
  • connector directory requests
• codex-rs/core/src/plugins/remote.rs
  • /plugins/list
  • /plugins/featured
  • /plugins/{id}/enable
  • /plugins/{id}/uninstall
• codex-rs/core-skills/src/remote.rs
  • /hazelnuts
  • /hazelnuts/{skill_id}/export
• codex-rs/analytics/src/client.rs
  • /codex/analytics-events/events
• codex-rs/core/src/arc_monitor.rs
  • /codex/safety/arc
• codex-rs/app-server/src/transport/remote_control/enroll.rs
  • /wham/remote/control/server/enroll
• codex-rs/app-server/src/transport/remote_control/websocket.rs
  • websocket handshake for /wham/remote/control/server
• codex-rs/cloud-tasks/src/util.rs
• codex-rs/cloud-tasks/src/env_detect.rs
  • /wham/environments
  • /wham/environments/by-repo/...

One backend hit I found that does not look like part of this auth migration is:

• codex-rs/core/src/plugins/startup_sync.rs
  • backup archive fetch at /backend-api/plugins/export/curated

So the main point is: this PR fixes the shared model path, but it does not yet migrate all of the places where we talk to codex-backend.