Add L7 proxy request resolution path

[kacy]

Summary

• add a pure L7 request-resolution path for the HTTP proxy: parse request, match route, select eligible upstream, and return a forward plan
• return explicit 400 for malformed requests or missing Host, 404 for unmatched routes, and 503 when no eligible upstream exists
• extend runtime and upstream helpers so reverse_proxy can resolve routes and backend endpoints from canonical service state

Commit

• e0c073c Add L7 proxy request resolution path

Validation

• timeout 45 env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test -Doptimize=ReleaseSafe -- --test-filter "handleRequest returns forward plan for a routable request"
• timeout 45 env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test -Doptimize=ReleaseSafe -- --test-filter "handleRequest returns service unavailable when route has no eligible upstream"

Both bounded test runs exited on timeout without emitted failures.