feat(nemo-agents-plugin): add codex adapter to NAT plugin

[mmogallapalli]

PR Description

Summary

Adds experimental Codex agent support to nemo-agents by vendoring the NAT
Codex adapter and wiring it into the plugin package.

This PR:

• Updates NAT dependencies to the 1.8.x family
• Vendors the upstream experimental Codex adapter into plugins/nemo-agents
• Exposes the adapter through the nat.components entry point
• Recognizes codex_agent in nemo-agents config validation
• Adds a Codex example config and README
• Adds tests for adapter registration and example config validation

Why vendor the Codex adapter?

The Codex adapter exists upstream in the NeMo Agent Toolkit repo under
examples/experimental/codex_agent_adapter, but it is not currently published
as an installable package that end users can depend on directly.

That means a normal user installation like:

uv pip install plugins/nemo-agents/

would not include the Codex adapter unless users also cloned the NAT repo and
manually installed the experimental example package. That is not a viable
product path for nemo-agents.

Vendoring gives us a shippable path now:

• nemo-agents installs with Codex adapter support included
• Users do not need a local NAT source checkout
• The adapter is discoverable through normal NAT plugin entry points
• The vendored source is pinned to an explicit upstream NAT commit for traceability
• This follows the same general pattern already used for vendored Switchyard code

The vendored source records its upstream provenance in:

plugins/nemo-agents/vendor/codex_agent_adapter/README.md

User Flow

Users install/use nemo-agents as usual, then provide a config with:

workflow:
  _type: codex_agent

The included example is:

plugins/nemo-agents/examples/codex-agent/codex-agent.yml

The Codex adapter requires codex and nemo-relay to be installed on PATH.
Relay is used internally by the adapter to run Codex and bridge Codex events
into NAT telemetry.

Validation

Focused tests:

uv run --frozen pytest plugins/nemo-agents/tests/unit/test_codex_adapter_packaging.py plugins/nemo-agents/tests/unit/test_container.py -q

Result:

101 passed, 1 warning

Manual smoke tests:

• nemo agents package --no-build validates the Codex example without unknown workflow warnings
• nemo agents create --name codex-agent --agent-config plugins/nemo-agents/examples/codex-agent/codex-agent.yml
• nemo agents deploy --agent codex-agent
• nemo agents invoke --agent codex-agent --input "Read pyproject.toml and say only the project name. Do not edit files."

Invoke returned:

nemoplatform

[github-actions]

Suite	Lines Covered	Line Rate	Branch Rate
Unit Tests	21322/27924	76.4%	61.4%
Integration Tests	12350/26693	46.3%	19.7%