test(ingestion): OIDC discovery 502s then 200 against a real issuer (#83)

The retry landed in 9a232ad but was tested only through startupretry with
fake funcs; nothing exercised chassisauth.NewJWTValidator's real 502 path.
Extracts the main() closure into buildJWTValidator (behaviour unchanged)
and tests it against an httptest issuer: 3x502 then 200 succeeds with 4
discoveries; a persistent 502 stops at exactly the 8-attempt budget with
the last error; cancelling ctx mid-backoff returns context.Canceled.
Flip-verified: Attempts=1 turns all three red.

Lane: risk/medium issue, committed to main because the behaviour change
already shipped in 9a232ad; this is a test plus a pure extraction.

Closes #83

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/cmd/server/main.go

@@ -381,15 +381,9 @@ func main() {
 	//
 	// The budget stays bounded: retrying forever would turn a misconfiguration
 	// into a process that looks alive and never serves.
-	jwtValidator, err := startupretry.Do(ctx,
-		startupretry.Policy{Attempts: 8, Base: time.Second, Max: 30 * time.Second},
-		func(ctx context.Context) (*chassisauth.JWTValidator, error) {
-			v, err := chassisauth.NewJWTValidator(ctx, os.Getenv("DEX_ISSUER_URL"), os.Getenv("MCP_AUDIENCE"))
-			if err != nil {
-				logger.Warn("oidc discovery not ready, retrying", "err", err)
-			}
-			return v, err
-		})
+	jwtValidator, err := buildJWTValidator(ctx, logger,
+		os.Getenv("DEX_ISSUER_URL"), os.Getenv("MCP_AUDIENCE"),
+		startupretry.Policy{Attempts: 8, Base: time.Second, Max: 30 * time.Second})
 	if err != nil {
 		logger.Error("build jwt validator", "err", err)
 		os.Exit(1)