Add nexus API integration as astro nexus subcommands

[feluelle]

Description

Describe the purpose of this pull request.

Register dynamic API commands from the nexus shared library under astro nexus <command>. Includes automatic flag/body field parsing from apis.json schema, positional argument defaults from config, inline status updates on stderr for long-running commands (suppressed when piping), and shell completion support.

🎟 Issue(s)

Related #XXX

🧪 Functional Testing

List the functional testing steps to confirm this feature or fix.

📸 Screenshots

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📋 Checklist

• Rebased from the main (or release if patching) branch (before testing)
• Ran make test before taking out of draft
• Ran make lint before taking out of draft
• Added/updated applicable tests
• Tested against Astro-API (if necessary).
• Tested against Houston-API and Astronomer (if necessary).
• Communicated to/tagged owners of respective clients potentially impacted by these changes.
• Updated any related documentation