Add a DNS resolution timeout and AbortController to lookup() in src/webhooks/ssrfGuard.ts

[attyolu]

##closes #485

# fix(ssrf): add DNS lookup timeout and abort to webhook target validation

## Description
This PR resolves issue #485 by adding a configurable DNS resolution timeout to the SSRF guard `validateWebhookTarget`. 

Currently, the SSRF guard calls `dns.promises.lookup(hostname, { all: false })` without a deadline. If a target resolver is slow, unresponsive, or maliciously configured, it can hang indefinitely, stalling the webhook validation path and blocking caller delivery threads.

## Key Changes

### 1. Configuration
- Added the `WEBHOOK_DNS_TIMEOUT_MS` environment variable (default: `2000` ms) to the environment schema in `src/config/env.ts`.
- Added schema tests in `src/config/env.test.ts` to verify default behavior and ensure configuration bounds are correctly enforced.

### 2. SSRF Guard Logic & Fail-Closed Behavior
- Implemented `lookupWithTimeout` helper function in `src/webhooks/ssrfGuard.ts` which wraps `dns.promises.lookup` using a `Promise.race` timeout and an `AbortController`.
- Pass the `AbortSignal` to Node's `dns.promises.lookup`. If a timeout is hit, the controller signals an abort (cancelling the pending lookup to prevent memory/resource leaks) and rejects the target with a `'DNS_TIMEOUT'` validation error code.
- Added specific error codes (e.g. `'DNS_TIMEOUT'`, `'BLOCKED_ADDRESS'`) to `WebhookTargetValidationError` to easily distinguish DNS timeout events from private address rejections.

### 3. Unit Testing
- Added `tests/webhooks/ssrfGuard.test.ts` asserting:
  - Timeout fail-closed behavior when a DNS lookup hangs.
  - That the controller successfully aborts the signal on timeout to prevent leaking lookups.
  - Fail-closed behavior on lookup errors (e.g. `ENOTFOUND`).
  - Standard public and private IP resolution validations.

### 4. Documentation
- Documented `WEBHOOK_DNS_TIMEOUT_MS` in the configuration tables of `README.md` and `docs/webhooks.md`.
- Documented DNS resolution timeout details and error responses in `docs/webhooks.md`.

## Security Notes
- **Fail-Closed Semantics**: A DNS timeout throws a validation error and rejects the target as unsafe, ensuring we fail-closed rather than allowing delivery.
- **Resource Protection**: The lookup is aborted via Node's native cancellation support (`AbortSignal`) to ensure resources are not leaked during timeouts.

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[Jagadeeshftw]

important security hardening - the ssrf guard was calling dns.promises.lookup without any deadline, so a slow or malicious resolver could hang webhook validation indefinitely. the lookupWithTimeout helper uses Promise.race with an AbortController so the lookup is actually cancelled on timeout (not just abandoned), and the fail-closed behavior means a dns timeout rejects the target rather than letting it through. WEBHOOK_DNS_TIMEOUT_MS defaulting to 2000ms is a sensible conservative value.