TUN-10385: Add connectivity checks foundation

This adds the foundation for the new cloudflared pre-checks by creating a new prechecks package. This adds the following:

* types.go: Status, CheckResult, Report, Config (add IPVersion allregions.ConfigIPVersion field to Config)
* interfaces.go: DNSResolver, TCPDialer, QUICDialer, ManagementDialer

prechecks/types.go

@@ -0,0 +1,94 @@
+package prechecks
+
+import (
+	"time"
+
+	"github.com/cloudflare/cloudflared/connection"
+	"github.com/cloudflare/cloudflared/edgediscovery/allregions"
+)
+
+// Status represents the outcome of a single connectivity pre-check.
+type Status int
+
+const (
+	// Pass indicates the check completed successfully.
+	Pass Status = iota
+	// Warn indicates a soft failure: cloudflared can still run but in a
+	// degraded state (e.g. one transport blocked, API unreachable).
+	Warn
+	// Fail indicates a check failure that the user should act on (e.g.
+	// DNS unresolvable, both transports blocked). cloudflared still starts;
+	// this status is purely informational.
+	Fail
+	// Skip indicates the check was not executed because a prerequisite
+	// check (typically DNS) failed first.
+	Skip
+)
+
+// String returns the canonical display name for a Status value.
+func (s Status) String() string {
+	switch s {
+	case Pass:
+		return "PASS"
+	case Warn:
+		return "WARN"
+	case Fail:
+		return "FAIL"
+	case Skip:
+		return "SKIP"
+	default:
+		return "UNKNOWN"
+	}
+}
+
+// CheckResult holds the outcome of one individual connectivity probe.
+type CheckResult struct {
+	// Component is the human-readable probe category shown in the table header
+	// column, e.g. "DNS Resolution", "QUIC Connectivity".
+	Component string
+
+	// Target is the address or resource that was probed, e.g.
+	// "region1.v2.argotunnel.com" or "Port 7844 (QUIC)".
+	Target string
+
+	// ProbeStatus is the outcome of the probe.
+	ProbeStatus Status
+
+	// Details is a short description of the result shown in the table, e.g.
+	// "Resolved successfully" or "Handshake failed".
+	Details string
+
+	// Action is non-empty when ProbeStatus is Warn or Fail and contains
+	// a human-readable remediation instruction, e.g.
+	// "Allow outbound QUIC on port 7844."
+	Action string
+}
+
+// Report aggregates all CheckResults produced by a single Run() invocation.
+// Pre-checks run in parallel with tunnel initialization and are purely
+// diagnostic: the Report is displayed to the user but never gates startup.
+type Report struct {
+	// Results contains one entry per executed probe, in the order they were
+	// collected.
+	Results []CheckResult
+
+	// SuggestedProtocol is the connection protocol the pre-checks recommend
+	// based on transport probe results.
+	SuggestedProtocol connection.Protocol
+}
+
+// Config controls the behavior of a pre-check Run().
+type Config struct {
+	// Region is the optional cloudflared --region flag value. When non-empty
+	// the pre-check probes the regional edge hostnames instead of the global ones.
+	Region string
+
+	// Timeout is the maximum wall-clock duration allowed for the entire
+	// pre-check suite to complete.
+	Timeout time.Duration
+
+	// IPVersion controls which address families are probed for transport
+	// checks. It mirrors the --edge-ip-version CLI flag so that the pre-check
+	// exercises the same code paths the tunnel itself will use.
+	IPVersion allregions.ConfigIPVersion
+}