Fix #83: httpserver HandleGroup should mount both group root and subtree

[mishankov]

Closes #83

Summary

httpserver.HandlerGroup.HandleGroup(pattern, handler) previously registered only pattern + "/" and applied http.StripPrefix(pattern, handler). This meant a mounted group did not handle its exact root path unless the caller also registered the same handler with Handle(pattern, handler).

This change updates HandleGroup to mount the group at both:

• /pattern -> nested handler receives /
• /pattern/... -> nested handler receives /...

What changed

• handlergroup.go:

  • HandleGroup now normalizes the mount pattern (trims trailing /) and registers both exact root and slash subtree routes.
  • Added stripPrefix helper that behaves like http.StripPrefix, but maps an empty stripped URL.Path to / so nested groups can naturally register GET / etc.
  • stripPrefix also propagates URL.RawPath for parity with standard library behavior.

• httpserver_test.go:

  • Added regression test for exact group root routing (/hg -> GET /).
  • Added regression test for subtree routing (/domains/verify -> GET /verify).
  • Added test for trailing-slash pattern normalization (HandleGroup("/hg/", ...) behaves like HandleGroup("/hg", ...)).
  • Added negative test for 404 when prefix does not match.

Testing / validation

• go test ./httpserver/... — passed
• task check (lint + race tests) — passed with 0 issues

Code review summary

• P1 blocker found and fixed: Initial implementation of stripPrefix only rewrote URL.Path but ignored URL.RawPath, creating a behavior regression vs. http.StripPrefix. Fixed by also stripping RawPath when non-empty.
• P2 gaps addressed: Added tests for trailing-slash pattern normalization and 404 on prefix mismatch.
• No remaining P0/P1 blockers.

Known limitations / follow-ups

None.