fix(security): fail closed when an agent-conversation webhook secret is unset

[massimoalbarello]

The agent-conversations webhook verified its shared secret only when one happened to be configured:

// nango-integrations/agent-conversations/syncs/conversations.ts:58 (before)
if (metadata.webhookSecret && payload.secret !== metadata.webhookSecret) {
  throw new Error('Invalid agent conversation webhook secret');
}

If a connection's metadata carried no webhookSecret, the guard short-circuited and every payload was accepted. MetadataSchema declared the field .optional(), so that state was well-formed rather than a parse error.

Why it matters — this sync is reachable from Nango's public ingress: nango/packages/server/lib/routes.public.ts:183 registers /webhook/:environmentUuid/:providerConfigKey on publicAPI behind only a rate limiter. Against a secret-less connection, anyone who knows the environment UUID and provider config key can push arbitrary AgentConversation records straight into brain.records, which then feed the knowledge-distillation pipeline — so this is a knowledge-poisoning path, not just spurious rows.

Scope of exposure — the deployed connections are not currently affected. _scripts/lib/bootstrap-connections.ts:27 reads the secret via requiredValue(), which throws on a missing or empty AGENT_SYNC_WEBHOOK_SECRET, so every bootstrapped connection has one. The gap opens only for connections created out-of-band (Nango UI or API). I'm treating "the deploy script happens to set it" as an accidental mitigation rather than a control, which is why this fixes the check itself.

Fix — webhookSecret becomes required (.min(1)), so parseMetadata throws before any record is written when it is absent, and the comparison is now unconditional.

Verified by reverting the source change and re-running the new tests: the no-secret case saves records against the old code and rejects against the new one. The wrong-secret test passes either way and is there as a regression guard.

Note this is intentionally a breaking change for any existing secret-less connection — it will now error on webhook delivery until a secret is set, which is the fail-closed behaviour. Constant-time comparison was deliberately left out: node:crypto is unused anywhere in nango-integrations and a remote timing attack on this value isn't practical, so pulling it in would have widened the diff for no real gain.