feat: layer code-driven plugin config

[zhongxuanwang-nv]

Overview

Layers code-driven plugin configuration over materialized file-backed plugin configuration so callers can combine discovered plugins.toml, inline [plugins].config, CLI overrides, request headers, and binding-level config builders deterministically.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Added layer_plugin_config to the Rust core plugin API with recursive object merging, scalar/array replacement, and top-level component upsert by kind.
• Exposed plugin config layering across FFI, Python, Node.js, WebAssembly, and Go wrapper surfaces.
• Updated CLI config resolution so file-backed config remains compatible while code-driven layers like --plugin-config and request header config overlay the effective base.
• Added effective plugin config source tracking in gateway config, doctor output, launcher printing, and server activation errors.
• Updated plugin configuration docs and agent skills to describe precedence, file-source conflicts, and code-driven overlays.
• Added focused tests across core, CLI, FFI, Python, Node.js, Go, and WebAssembly.

Quick functional check:

from nemo_relay import plugin

base = {
    "version": 1,
    "components": [
        {
            "kind": "observability",
            "enabled": False,
            "config": {
                "atof": {
                    "enabled": True,
                    "filename": "base.jsonl",
                }
            },
        }
    ],
}
overlay = {
    "components": [
        {
            "kind": "observability",
            "config": {
                "atof": {
                    "mode": "overwrite",
                }
            },
        }
    ],
}

plugin.layer(base, overlay)

Expected output:

{
    "version": 1,
    "components": [
        {
            "kind": "observability",
            "enabled": False,
            "config": {
                "atof": {
                    "enabled": True,
                    "filename": "base.jsonl",
                    "mode": "overwrite",
                }
            },
        }
    ],
}

Validation: full Rust, Python, Node.js, Go, WebAssembly, docs, cargo fmt, cargo clippy --workspace --all-targets -- -D warnings, and uv run pre-commit run --all-files passed. Focused reruns covered core layering, CLI layer precedence, the real nemo-relay run --dry-run binary path, and the Python/Node.js/Go/FFI/WebAssembly wrapper paths.

Where should the reviewer start?

Start with crates/core/src/plugin.rs for the shared layering semantics, then crates/cli/src/config.rs for source precedence and CLI/header composition.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to: RELAY-183 (https://linear.app/nvidia/issue/RELAY-183/layer-code-driven-plugin-config-over-materialized-file-configs)

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[github-actions]

Fern docs preview: https://nvidia-preview-pull-request-199.docs.buildwithfern.com/nemo/relay (​https://nvidia-preview-pull-request-199.docs.buildwithfern.com/nemo/relay​)