exec-server: support auth-backed remote executor registration (#22769)

This updates remote `exec-server` registration to use normal Codex auth
instead of a registry-issued credential. The registry request is built
from the existing auth-provider path, which preserves the biscuit-only
registry contract introduced in
[openai/openai#924101](https://github.com/openai/openai/pull/924101)
while removing the old remote registry bearer env var and its direct
transport assumptions.

The default remote flow uses persisted ChatGPT auth from the normal
Codex config/storage path. This PR also includes the containerized Agent
Identity path needed by
[openai/openai#924260](https://github.com/openai/openai/pull/924260):
remote `exec-server` accepts `--allow-agent-identity-auth`, permits
Agent Identity auth loaded from `CODEX_ACCESS_TOKEN` only when that flag
is present, and reuses the existing Agent task registration plus derived
`AgentAssertion` header generation. API-key auth remains unsupported,
and Agent Identity stays opt-in.

Validation performed beyond normal presubmit coverage:
- `cargo fmt --all --check`
- `cargo check -p codex-cli`
- `cargo test -p codex-exec-server`
- `cargo test -p codex-cli exec_server_agent_identity_auth_flag_`
- `cargo test -p codex-cli remote_exec_server_auth_mode_`

I also attempted `cargo test -p codex-cli`. The new CLI tests passed
inside that run, but the suite ended on an unrelated local
marketplace-state failure in
`plugin_list_excludes_unconfigured_repo_local_marketplaces`.

codex-rs/Cargo.lock

@@ -2748,6 +2748,7 @@ dependencies = [
  "axum",
  "base64 0.22.1",
  "bytes",
+ "codex-api",
  "codex-app-server-protocol",
  "codex-client",
  "codex-file-system",
@@ -2759,6 +2760,7 @@ dependencies = [
  "codex-utils-rustls-provider",
  "ctor 0.6.3",
  "futures",
+ "http 1.4.0",
  "pretty_assertions",
  "prost 0.14.3",
  "reqwest",