fix(agent): Implement BGP uplink health checks for NVUE REST client

[DrewBloechl]

This is a backport to v2.1 of #5075; the original text of that merge follows:

This implements the BGP uplink health checks that didn't get ported over when I did the initial NVUE REST client work for DPF. This breaks down like so:

• Add new NvueClient methods to fetch per-VRF BGP data (using the OpenAPI spec to generate the types).
• Add health::nvue::check_bgp_uplink_sessions to implement health checks from the above BGP data.
• Rework the NVUE REST health checks to call this after checking whether the REST API is up.

Related issues

• Internal NVBugs ID 6563638

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

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_{🕐 Last updated: 2026-08-18 16:55:17 UTC | Commit: 9a5e6e3}

[nv-dmendoza]

LGTM, backports needed fix