[codex] Use AgentAssertion downstream behind use_agent_identity (#17980)

## Summary

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

It isolates task-scoped downstream AgentAssertion wiring on top of the
merged PR3.1 work without re-carrying the earlier agent registration,
task registration, or task-state history.

This PR includes the task-scoped bug-fix call sites from the review:
generic file upload auth, MCP OpenAI file upload auth, and ARC monitor
auth. Broader user/control-plane calls move to PR4.1 and PR4.2.

## Stack

- PR1: https://github.com/openai/codex/pull/17385 - add
`features.use_agent_identity`
- PR2: https://github.com/openai/codex/pull/17386 - register agent
identities when enabled
- PR3: https://github.com/openai/codex/pull/17387 - register agent tasks
when enabled
- PR3.1: https://github.com/openai/codex/pull/17978 - persist and
prewarm registered tasks per thread
- PR4: this PR - use task-scoped `AgentAssertion` downstream when
enabled
- PR4.1: https://github.com/openai/codex/pull/18094 - introduce
AuthManager-owned background/control-plane `AgentAssertion` auth
- PR4.2: https://github.com/openai/codex/pull/18260 - use background
task auth for additional backend/control-plane calls

## What Changed

- add AgentAssertion envelope generation in `codex-core`
- route downstream HTTP and websocket auth through AgentAssertion when
an agent task is present
- extend the model-provider auth provider so non-bearer authorization
schemes can be passed through cleanly
- make generic file uploads attach the full authorization header value
- make MCP OpenAI file uploads use the cached thread agent task
assertion when present
- make ARC monitor calls use the cached thread agent task assertion when
present

## 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 check -p codex-core -p codex-login -p codex-analytics -p
codex-app-server -p codex-cloud-requirements -p codex-cloud-tasks -p
codex-models-manager -p codex-chatgpt -p codex-model-provider -p
codex-mcp -p codex-core-skills`
- `cargo test -p codex-model-provider bearer_auth_provider`
- `cargo test -p codex-core agent_assertion`
- `cargo test -p codex-app-server remote_control`
- `cargo test -p codex-cloud-requirements fetch_cloud_requirements`
- `cargo test -p codex-models-manager manager::tests`
- `cargo test -p codex-chatgpt`
- `cargo test -p codex-cloud-tasks`
- `cargo test -p codex-login agent_identity`
- `just fix -p codex-core -p codex-login -p codex-analytics -p
codex-app-server -p codex-cloud-requirements -p codex-cloud-tasks -p
codex-models-manager -p codex-chatgpt -p codex-model-provider -p
codex-mcp -p codex-core-skills`
- `just fix -p codex-app-server`
- `git diff --check`

codex-rs/core/src/agent_identity/task_registration.rs

@@ -2,6 +2,7 @@ use std::time::Duration;
 
 use anyhow::Context;
 use anyhow::Result;
+use codex_login::AgentTaskAuthorizationTarget;
 use codex_protocol::protocol::SessionAgentTask;
 use crypto_box::SecretKey as Curve25519SecretKey;
 use ed25519_dalek::Signer as _;
@@ -102,6 +103,13 @@ impl AgentIdentityManager {
 }
 
 impl RegisteredAgentTask {
+    pub(crate) fn authorization_target(&self) -> AgentTaskAuthorizationTarget<'_> {
+        AgentTaskAuthorizationTarget {
+            agent_runtime_id: &self.agent_runtime_id,
+            task_id: &self.task_id,
+        }
+    }
+
     pub(crate) fn to_session_agent_task(&self) -> SessionAgentTask {
         SessionAgentTask {
             agent_runtime_id: self.agent_runtime_id.clone(),