Discovered-mode outgoing auth: should an explicit per-backend override win over discovered auth?

Context

Split out of the #6242 review, where this was (rightly) flagged as a behaviour change that should not be settled as a side effect of a PR about reporting unsupported auth types.

Current behaviour

With VirtualMCPServer.spec.outgoingAuth.source: discovered, auth discovered from the backend resource's own externalAuthConfigRef wins over an explicit per-backend entry in spec.outgoingAuth.backends. The explicit entry only takes effect as a fallback when the backend has no discovered auth (pkg/vmcp/aggregator/discoverer.go, applyAuthConfigToBackend).

Question

Is that the intended precedence? An operator who writes an explicit entry for a specific named backend arguably expresses stronger intent than the backend's own declaration — that is how per-backend overrides read elsewhere in the config. On the other hand, the backend resource's declaration is maintained closest to the backend, and letting the vMCP spec override it means a vMCP author can point a backend at a different credential than its owner configured — which may be exactly the point, or exactly the problem, depending on the trust model between the two roles.

Whichever way this lands, it decides which credential a live backend authenticates with, so it deserves its own decision (and probably a note in the VirtualMCPServer docs about which side wins and why). The first revision of #6242 implemented override-wins together with an ExplicitBackends marker to distinguish operator-written entries from discovery-populated ones in the generated config; that implementation was reverted per review but can serve as a reference point.

Related

• #6242 (review discussion)
• #5930 (the unsupported-type reporting this was split from)

Start in pkg/vmcp/aggregator/discoverer.go at applyAuthConfigToBackend, then read the linked #6242 review and #5930 for the existing behavior and reverted approach. Done means a precedence decision is recorded, the selected behavior is covered in the relevant implementation, and the VirtualMCPServer documentation explains which auth declaration wins and why.