Remove the Router abstraction from v2/httpserver per the consensus
reached in team-tasks#4311 and formalized in ADR 001 (handbook !18825).

LabKit does not own the routing layer. Consumers bring their own router
(stdlib ServeMux, chi, or any http.Handler) and pass it via
Config.Handler. Built-in middleware (tracing, access logging) wraps the
handler automatically.

Key changes:

- Remove Router interface (17 methods) and chiRouter adapter
- Replace Config.Router with Config.Handler http.Handler
- Remove Server.Router() accessor method
- URLParam now delegates to r.PathValue() (Go 1.22+), works with both
  stdlib and chi v5.1+
- Add SetRoutePattern/RoutePattern context helpers for low-cardinality
  span names. Uses a mutable holder pattern so inner middleware can
  communicate back to the outer tracing middleware.
- Remove github.com/go-chi/chi/v5 dependency from v2/go.mod

Closes team-tasks#4294
Relates to team-tasks#4311

The function was only used by the old Router.Use() path. Now that
NewWithConfig wraps the handler directly with AccessLogger, this
helper is dead code. Caught by golangci-lint (unused).

The postgres integration test has no dependency on other validate jobs
(no artifact consumption, independent services). Running it in a
separate integration stage forced it to wait for all 43 validate jobs
to complete first, adding unnecessary wall-clock time to the pipeline.

Move it to the validate stage so it runs in parallel with all other
jobs. Remove the now-unused integration stage.

This reverts commit 97b637f0.

refactor(v2/httpserver): remove Router interface, accept http.Handler

Closes gitlab-org/quality/quality-engineering/team-tasks#4294

See merge request !369



Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: dbarrett <dbarrett@gitlab.com>