feat: add runner health admission checks (#1090)

Opt-in local task-admission checks under a `health_check` config section (disabled by default):

- pause new task fetching when free disk space on the workspace volume is below the configured minimum
- optional executable health-check script — a non-zero exit, timeout, or start failure marks the runner unavailable
- checks run only while the runner is idle; the last result is reused while a job is active, and polling resumes automatically on recovery
- `/readyz` reports task-admission readiness (reusing the poll loop's last check); `/healthz` stays a process-liveness endpoint

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1090
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/metrics/server_test.go

@@ -0,0 +1,43 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package metrics
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestReadyEndpoint(t *testing.T) {
+	tests := []struct {
+		name   string
+		ready  bool
+		reason string
+		status int
+	}{
+		{"ready", true, "ok", http.StatusOK},
+		{"not ready", false, "low disk space", http.StatusServiceUnavailable},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			handler := NewHTTPHandler(func() (bool, string) { return test.ready, test.reason })
+			request := httptest.NewRequest(http.MethodGet, "/readyz", nil)
+			response := httptest.NewRecorder()
+			handler.ServeHTTP(response, request)
+			assert.Equal(t, test.status, response.Code)
+			assert.Equal(t, test.reason, response.Body.String())
+		})
+	}
+}
+
+func TestHealthEndpointStaysLiveWhenNotReady(t *testing.T) {
+	handler := NewHTTPHandler(func() (bool, string) { return false, "low disk space" })
+	request := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	response := httptest.NewRecorder()
+	handler.ServeHTTP(response, request)
+	assert.Equal(t, http.StatusOK, response.Code)
+	assert.Equal(t, "ok", response.Body.String())
+}