Replace gorilla/mux with Go 1.22 stdlib ServeMux in test server

[pietern]

Summary

• Remove the gorilla/mux dependency by switching libs/testserver to Go 1.22's enhanced net/http.ServeMux, which added method-based routing ("GET /items/{id}"), path wildcards ({id}), and rest wildcards ({path...}) — the same features we used gorilla for. See the Go 1.22 release notes.
• Replace gorilla's {path:.*} regex wildcards with stdlib {path...} rest wildcards, and mux.Vars(r) with r.PathValue().
• Exact (non-wildcard) paths are dispatched via a map lookup before reaching ServeMux. This avoids a ServeMux limitation where registering method-specific exact paths alongside method-specific wildcard patterns panics due to Go's implicit GET→HEAD handling creating unresolvable precedence. When an exact path has no handler for the request method, it falls through to ServeMux where a wildcard handler can serve it (e.g., a stub registers GET /exact/path, but HEAD falls through to a wildcard HEAD handler).
• Wildcard patterns in test.toml stubs must now use the same placeholder names as the default handlers they coexist with — ServeMux panics on structurally identical patterns with different wildcard names. Two stubs updated: {name} → {full_name}.

Test plan

• Unit tests pass (libs/testserver, bundle/direct/dresources, acceptance/internal)
• Acceptance selftests pass
• Full CI

This pull request was AI-assisted by Isaac.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

Files: acceptance/bundle/invariant/test.toml, acceptance/bundle/resources/synced_database_tables/basic/test.toml
Suggested: @denik
Also eligible: @shreyas-goenka, @andrewnester, @anton-107, @janniklasrose, @lennartkats-db

General files (require maintainer)

Files: NOTICE, acceptance/internal/prepare_server.go, go.mod, go.sum, libs/testserver/handlers.go, libs/testserver/server.go
Based on git history:

• @denik -- recent work in libs/testserver/, acceptance/bundle/invariant/, acceptance/internal/

_{Any maintainer (@andrewnester, @anton-107, @denik, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
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