Split Raft::HTTP::Handler into StatusHandler and AdminHandler

[antondalgren]

Summary

The old `Handler` bundled read-only GETs (`/raft/status`, `/raft/log`, `/raft/metrics`) with mutating `POST /raft/admin/*` in a single handler. Consumers that want to expose metrics on an unauthenticated port were forced to expose unauthenticated cluster-mutating admin endpoints alongside.

Replace with two handlers that can be mounted separately:

• `StatusHandler(T)` — read-only routes (`GET /raft/status`, `/raft/log`, `/raft/metrics`). Anything else falls through via `call_next`. Safe on an unauthenticated metrics port.
• `AdminHandler(T)` — `POST /raft/admin/*` including the `raft_debug`-flagged pause/resume/partition/heal/reset routes. Anything else falls through. Mount behind authentication.

No auth is added to the shard itself — auth is consumer policy. The two halves can be combined into one mount with `[StatusHandler, AdminHandler]` to preserve current behaviour where needed.

Behaviour is byte-for-byte identical to the old handler: same JSON shapes, status codes, error messages, and `JSON::ParseException | KeyError | ArgumentError` rescue around admin routes.

Changes

• `src/raft/http/status_handler.cr` — new, GET-only routes
• `src/raft/http/admin_handler.cr` — new, POST admin routes
• `src/raft/http/handler.cr` — deleted, no backwards-compat shim (pre-1.0)
• `src/raft.cr` — requires both new files
• `examples/kv/src/main.cr`, `examples/queue/src/main.cr` — chain both handlers (drop-in replacement for the old single handler)
• `spec/raft/http/{status,admin}_handler_spec.cr` — split tests with new cases:
  • StatusHandler does NOT respond to `POST /raft/admin/...` (falls through; bare chain → 404) — proves the metrics-port mount cannot mutate
  • AdminHandler does NOT respond to `GET /raft/status` (falls through)
  • A chain `[StatusHandler, AdminHandler]` serves both route sets — parity with the old combined handler
  • `raft_debug`-gated pause/resume coverage moved to admin spec

LavinMQ note

LavinMQ is the sole external consumer. It instantiates `Raft::HTTP::Handler` in one place per integration. The upgrade is mechanical: replace the single instantiation with `StatusHandler` + `AdminHandler` mounted on whichever ports their auth chain dictates.

Test plan

• `crystal spec spec/raft/ -Dpreview_mt -Dexecution_context` — 118/118
• `crystal spec spec/raft/ -Dpreview_mt -Dexecution_context -Draft_debug` — 125/125
• `crystal build --no-codegen --error-on-warnings` on lib + both examples — clean
• `crystal tool format --check src/ spec/` — clean
• `lib/ameba/bin/ameba --fail-level error src/ spec/` — exit 0 (54 pre-existing warnings, none in new files)

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