Fix Heroku connection probe Accept header

Heroku's connection-status probe used a static ProbeURL, which the
generic probe issues with `Accept: application/json`. Heroku negotiates
the API version through the Accept media type and returns 400 for an
unversioned request, which doProbeRequest reads as "connected" -- so the
probe never caught a revoked token (it only surfaced at the first
ListAccounts).

Probe via a probeHeroku closure that sends
`Accept: application/vnd.heroku+json; version=3` instead. Verified live:
a dead token returns 400 with application/json but 401 with the
versioned header, which doProbeRequest correctly maps to rejected.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/provider/probe.go

@@ -31,6 +31,7 @@ import (
 const (
 	anthropicAPIVersion     = "2023-06-01"
 	anthropicUsersProbeURL  = "https://api.anthropic.com/v1/organizations/users?limit=1"
+	herokuAccountProbeURL   = "https://api.heroku.com/account"
 	linearGraphQLEndpoint   = "https://api.linear.app/graphql"
 	mondayGraphQLEndpoint   = "https://api.monday.com/v2"
 	posthogOrganizationPath = "/api/organizations/@current/"
@@ -410,6 +411,26 @@ func probeAnthropic(
 	return doProbeRequest(httpClient, req)
 }
 
+func probeHeroku(
+	ctx context.Context,
+	httpClient *http.Client,
+	_ *coredata.Connector,
+) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, herokuAccountProbeURL, nil)
+	if err != nil {
+		return fmt.Errorf("cannot create probe request: %w", err)
+	}
+
+	// Heroku negotiates the API version through the Accept media type; the
+	// generic "application/json" the default probe sends yields 400 (not
+	// 401/403), which doProbeRequest would read as "connected" and mask a
+	// dead token. Send the versioned Accept so a revoked token surfaces as
+	// 401 (verified live: 400 with application/json, 401 with this header).
+	req.Header.Set("Accept", "application/vnd.heroku+json; version=3")
+
+	return doProbeRequest(httpClient, req)
+}
+
 func probePostHog(
 	ctx context.Context,
 	httpClient *http.Client,