fix(m2-pr-e): collapse AgentService.HeartbeatWithContext into Heartbeat

PR-E of 6 in the M-2 end-to-end remediation sequence. Collapses the
HeartbeatWithContext wrapper into a single ctx-first Heartbeat method,
matching D-1 (ctx-only signatures, no dual forms). The handler-facing
method name is preserved (D-4) — internal/api/handler/agents.go already
declares `Heartbeat(ctx, ...)` on its local service interface, and the
handler mock at internal/api/handler/agent_handler_test.go already
takes `_ context.Context` as its first param, so no handler churn.

Changes
-------
internal/service/agent.go
  - Delete the zero-body Heartbeat wrapper that forwarded to
    HeartbeatWithContext with context.Background().
  - Rename HeartbeatWithContext → Heartbeat (ctx-bearing body
    folded directly into the canonical method).

internal/service/agent_test.go
  - TestHeartbeat (L95) and TestHeartbeat_NotFound (L128):
    agentService.HeartbeatWithContext(ctx, ...) → .Heartbeat(ctx, ...).

internal/service/concurrent_test.go
  - L162: agentSvc.HeartbeatWithContext(ctx, agentID, metadata)
    → .Heartbeat(ctx, agentID, metadata).

internal/service/context_test.go
  - L179 + L232: agentSvc.HeartbeatWithContext(ctx, ...) → .Heartbeat(...)
  - L185 + L238 t.Logf strings: "HeartbeatWithContext with ..." →
    "Heartbeat with ..." to match the collapsed method name.

Verification (Go 1.25.9 linux/arm64, CI-parity caches)
------------------------------------------------------
  go build ./...                 clean
  go vet ./...                   clean
  go test -short ./internal/service/... ./internal/api/handler/... \
    ./internal/integration/...   all ok
  go test -race -short same set  all ok
  go test -short ./...           all packages ok
  golangci-lint run ./...        0 issues

Locked decisions from the M-2 plan:
  D-1 ctx-only signatures (no dual forms)
  D-4 preserve handler method names facing the router
  D-5 domain types stay ctx-free

Audit complete. Commit: 855124a9d9. Sections: 12. Findings: 2/7/10/4/6.

internal/service/agent.go

@@ -91,8 +91,8 @@ func (s *AgentService) Register(ctx context.Context, name string, hostname strin
 	return agent, apiKey, nil
 }
 
-// HeartbeatWithContext updates an agent's last seen time, status, and metadata.
+// Heartbeat updates an agent's last seen time, status, and metadata.
-func (s *AgentService) HeartbeatWithContext(ctx context.Context, agentID string, metadata *domain.AgentMetadata) error {
+func (s *AgentService) Heartbeat(ctx context.Context, agentID string, metadata *domain.AgentMetadata) error {
 	agent, err := s.agentRepo.Get(ctx, agentID)
 	if err != nil {
 		return fmt.Errorf("failed to fetch agent: %w", err)
@@ -114,12 +114,6 @@ func (s *AgentService) HeartbeatWithContext(ctx context.Context, agentID string,
 	return nil
 }
 
-// Heartbeat updates agent heartbeat (handler interface method).
-// Note: This method is called from handlers which have a context; callers should prefer HeartbeatWithContext.
-func (s *AgentService) Heartbeat(ctx context.Context, agentID string, metadata *domain.AgentMetadata) error {
-	return s.HeartbeatWithContext(ctx, agentID, metadata)
-}
-
 // SubmitCSR validates and processes a Certificate Signing Request from an agent.
 // In agent keygen mode, this completes an AwaitingCSR renewal job by signing the CSR
 // and storing the cert version. The private key stays on the agent — only the CSR

internal/service/agent_test.go

@@ -92,7 +92,7 @@ func TestHeartbeat(t *testing.T) {
 
 	agentService := NewAgentService(agentRepo, certRepo, jobRepo, targetRepo, auditService, issuerRegistry, nil)
 
-	err := agentService.HeartbeatWithContext(ctx, "agent-001", nil)
+	err := agentService.Heartbeat(ctx, "agent-001", nil)
 	if err != nil {
 		t.Fatalf("Heartbeat failed: %v", err)
 	}
@@ -125,7 +125,7 @@ func TestHeartbeat_NotFound(t *testing.T) {
 
 	agentService := NewAgentService(agentRepo, certRepo, jobRepo, targetRepo, auditService, issuerRegistry, nil)
 
-	err := agentService.HeartbeatWithContext(ctx, "nonexistent", nil)
+	err := agentService.Heartbeat(ctx, "nonexistent", nil)
 	if err == nil {
 		t.Fatal("expected error for nonexistent agent")
 	}

internal/service/concurrent_test.go

@@ -159,7 +159,7 @@ func TestConcurrentAgentHeartbeats(t *testing.T) {
 				Architecture: "x86_64",
 			}
 
-			err := agentSvc.HeartbeatWithContext(ctx, agentID, metadata)
+			err := agentSvc.Heartbeat(ctx, agentID, metadata)
 			if err != nil {
 				errChan <- fmt.Errorf("goroutine %d: failed heartbeat for agent %s: %w", idx, agentID, err)
 				return

internal/service/context_test.go

@@ -176,13 +176,13 @@ func TestAgentService_HeartbeatWithCancelledContext(t *testing.T) {
 		nil, // renewalService
 	)
 
-	err := agentSvc.HeartbeatWithContext(ctx, "agent-1", &domain.AgentMetadata{})
+	err := agentSvc.Heartbeat(ctx, "agent-1", &domain.AgentMetadata{})
 
 	// Service should handle cancelled context
 	if err == nil || ctx.Err() == context.Canceled {
 		return
 	}
-	t.Logf("HeartbeatWithContext with cancelled context returned: %v", err)
+	t.Logf("Heartbeat with cancelled context returned: %v", err)
 }
 
 // Test with timeout context (should trigger deadline exceeded)
@@ -229,11 +229,11 @@ func TestAgentService_HeartbeatWithDeadlineExceeded(t *testing.T) {
 
 	time.Sleep(10 * time.Millisecond) // Ensure deadline is exceeded
 
-	err := agentSvc.HeartbeatWithContext(ctx, "agent-1", &domain.AgentMetadata{})
+	err := agentSvc.Heartbeat(ctx, "agent-1", &domain.AgentMetadata{})
 
 	// Service should handle deadline exceeded
 	if err == nil || ctx.Err() == context.DeadlineExceeded {
 		return
 	}
-	t.Logf("HeartbeatWithContext with deadline exceeded returned: %v", err)
+	t.Logf("Heartbeat with deadline exceeded returned: %v", err)
 }