docs: document Event metadata injection

[ericevans-nv]

Overview

Documents the Event metadata injection middleware introduced in PR #800. The updated middleware guide explains how injectors are registered, when they run, what metadata they may add, and how failures and duplicate keys are handled.

• 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

This PR updates the middleware guide to cover:

• Global, scope-local, and native-plugin injector registration.
• Immutable Event context and insert-only metadata additions.
• Supported key and value formats.
• Deterministic priority and registration-name ordering.
• Callback error, panic, and invalid-output behavior.
• Processing order between internal transforms, metadata injection, sanitization, and subscriber delivery.
• Updated publication-flow descriptions and Mermaid diagrams.

This documentation depends on the Event metadata injection implementation in PR #800 and should merge after that implementation.

The full just docs build is currently unavailable because uv is missing from PATH. The commit-time Markdown link check will still run.

Where should the reviewer start?

Start with the Event Metadata Injection and Queued Event Publication sections in docs/about-nemo-relay/concepts/middleware.mdx.

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

• Relates to feat(core): add Event metadata injection middleware #800

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Fern docs preview: https://nvidia-preview-pull-request-801.docs.buildwithfern.com/nemo/relay/dev