fix(api): /auth/email/confirm-deletion JSON envelope (BUG-API-047/204/273)

The missing-token branch on /auth/email/confirm-deletion used to
`SendString("Missing token")` which served `Content-Type: text/plain`
with no envelope, no request_id, no agent_action — an envelope-bypass
that broke the universal {ok,error,message,request_id,agent_action}
contract every other 4xx in the api carries.

Agents grepping on `error: missing_token` (the canonical code already
used across onboarding.go, deletion_confirm.go:275, magic_link.go)
silently failed because this surface returned a plain string.

Route through respondError so the envelope shape + the existing
codeToAgentAction[missing_token] entry land here too.

The handler is browser-initiated (email-link click) but the response
shape must match the rest of the API for agent-driven probes (e.g. an
MCP tool unwrapping the JSON envelope before opening the dashboard).

Coverage block:
  Symptom:        /auth/email/confirm-deletion returns text/plain 400 with no
                  envelope (BUG-API-047, BUG-API-204, BUG-API-273)
  Enumeration:    rg -F 'SendString' internal/handlers/deletion_confirm.go (1 site)
                  rg -F 'confirm-deletion' internal/router/router.go (route bind)
  Sites found:    1 (deletion_confirm.go:510 SendString)
  Sites touched:  1
  Coverage test:  TestEmailConfirmDeletionRedirectHandler now asserts:
                   - status 400
                   - Content-Type application/json
                   - envelope.ok == false
                   - envelope.error == "missing_token"
                   - envelope.message non-empty
                   - envelope.agent_action non-empty (from codeToAgentAction)
                  so a future revert to SendString fails before merge.
  Live verified:  pending auto-deploy + curl -sI 'https://api.instanode.dev/auth/email/confirm-deletion' | grep -i content-type

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805

internal/handlers/deletion_confirm.go

@@ -507,7 +507,23 @@ func EmailConfirmDeletionRedirectHandler(dashboardBaseURL string) fiber.Handler
 	return func(c *fiber.Ctx) error {
 		token := c.Query("t")
 		if strings.TrimSpace(token) == "" {
-			return c.Status(http.StatusBadRequest).SendString("Missing token")
+			// BUG-API-047 / BUG-API-204 / BUG-API-273 (QA 2026-05-29):
+			// the missing-token branch used to SendString("Missing token")
+			// which served Content-Type: text/plain with no envelope,
+			// no request_id, no agent_action — an envelope-bypass that
+			// broke the universal {ok,error,message,request_id,agent_action}
+			// contract every other 4xx in the api carries. Agents grepping
+			// on `error: missing_token` (the canonical code already used
+			// across onboarding.go, deletion_confirm.go:275, magic_link.go)
+			// silently failed because this surface returned a plain string.
+			// Route through respondError so the envelope shape + the
+			// existing `missing_token` codeToAgentAction entry land here
+			// too. The handler is browser-initiated (email-link click) but
+			// the response shape needs to match the rest of the API for
+			// agent-driven probes (e.g. an MCP tool unwrapping the JSON
+			// envelope before opening the dashboard).
+			return respondError(c, http.StatusBadRequest, "missing_token",
+				"Sign-in link is missing its `t=...` token. Open the link from the email exactly as we sent it.")
 		}
 		// We deliberately encode the token as a query param on the
 		// dashboard URL so the dashboard's React router picks it up

internal/handlers/deletion_confirm_helpers_coverage_test.go

@@ -9,6 +9,9 @@ package handlers
 // white-box test exercises every branch deterministically.
 
 import (
+	"encoding/json"
+	"errors"
+	"io"
 	"net/http/httptest"
 	"strings"
 	"testing"
@@ -130,18 +133,64 @@ func TestShouldSkipEmailConfirmation_bvwave(t *testing.T) {
 
 func TestEmailConfirmDeletionRedirectHandler(t *testing.T) {
 	h := EmailConfirmDeletionRedirectHandler("https://dash.local/")
-	app := fiber.New()
+	// BUG-API-047/204/273: respondError returns ErrResponseWritten as a
+	// sentinel — fiber's default ErrorHandler would otherwise overwrite
+	// the JSON body with the sentinel string. Mirror the production
+	// router's swallow-sentinel handler so the assertions read the body
+	// the handler actually wrote.
+	app := fiber.New(fiber.Config{
+		ErrorHandler: func(c *fiber.Ctx, err error) error {
+			if errors.Is(err, ErrResponseWritten) {
+				return nil
+			}
+			return fiber.DefaultErrorHandler(c, err)
+		},
+	})
 	app.Get("/auth/email/confirm-deletion", h)
 
-	// Missing token → 400.
+	// BUG-API-047 / BUG-API-204 / BUG-API-273 (QA 2026-05-29):
+	// missing-token branch must return the canonical {ok,error,message,
+	// request_id,agent_action} envelope on Content-Type application/json,
+	// NOT text/plain "Missing token" which broke agents grepping on
+	// `error: missing_token`. Pin both the wire format and the canonical
+	// error code so a future revert to SendString fails this test.
 	resp, err := app.Test(httptest.NewRequest("GET", "/auth/email/confirm-deletion", nil))
 	if err != nil {
 		t.Fatal(err)
 	}
 	if resp.StatusCode != 400 {
 		t.Errorf("missing token status = %d; want 400", resp.StatusCode)
 	}
+	ct := resp.Header.Get("Content-Type")
+	if !strings.HasPrefix(ct, "application/json") {
+		t.Errorf("BUG-API-204/273: missing-token Content-Type = %q; want application/json (envelope, not text/plain)", ct)
+	}
+	body, _ := io.ReadAll(resp.Body)
 	resp.Body.Close()
+	var env struct {
+		OK          bool   `json:"ok"`
+		Error       string `json:"error"`
+		Message     string `json:"message"`
+		RequestID   string `json:"request_id"`
+		AgentAction string `json:"agent_action"`
+	}
+	if err := json.Unmarshal(body, &env); err != nil {
+		t.Fatalf("BUG-API-204/273: missing-token body not JSON: %v (body=%q)", err, string(body))
+	}
+	if env.OK {
+		t.Errorf("BUG-API-204/273: envelope.ok = true; want false on 400")
+	}
+	if env.Error != "missing_token" {
+		t.Errorf("BUG-API-047: envelope.error = %q; want %q (canonical code used across onboarding.go/magic_link.go/deletion_confirm.go)", env.Error, "missing_token")
+	}
+	if env.Message == "" {
+		t.Errorf("BUG-API-204/273: envelope.message empty")
+	}
+	// codeToAgentAction has a `missing_token` entry, so the envelope
+	// MUST carry a non-empty agent_action through respondError.
+	if env.AgentAction == "" {
+		t.Errorf("BUG-API-204/273: envelope.agent_action empty — codeToAgentAction[missing_token] should populate this")
+	}
 
 	// With token → 302 to the dashboard confirm page.
 	resp, err = app.Test(httptest.NewRequest("GET", "/auth/email/confirm-deletion?t=abc", nil))