caddyconfig: Register nested named-route invokes transitively

[SillyZir]

Fixes #6435.

Record which named routes each named route invokes during extraction and transitively attach those routes (with cycle protection) when populating a server's named_routes, so nested invoke targets are no longer dropped.

Previously, invoke foo inside a named route &(bar) was never registered into the server's named_routes, so invoking bar at runtime failed when it reached the nested invoke foo. The adapter now records, per named route, which named routes it invokes, and when a server references a named route it attaches that route plus everything it transitively invokes (breadth-first, with already-attached routes skipped so cycles terminate). The existing "cannot invoke named route which was not defined" error is preserved for undefined targets at any depth.

Files: caddyconfig/httpcaddyfile/httptype.go

Assistance Disclosure
This patch was developed with AI assistance. It was reviewed and tested before submission (affected package tests pass), and is submitted from a human-owned account that takes responsibility for the change.

[CLAassistant]

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