Add adapter-level alerting, alert dispatch, and customizable alert payloads

[Prateek32177]

Motivation

• Enable sending alerts directly from framework adapters and core queue flow so users can receive notifications whether or not they use Upstash controls.
• Allow consumers to override alert title and message while keeping sensible DLQ/non-DLQ fallbacks.
• Improve source/eventId resolution and avoid leaking undefined metadata fields when sending control alerts.

Description

• Introduces a new dispatchWebhookAlert helper (src/notifications/dispatch.ts) and wires adapter-level alerts + alert options into nextjs, express, and cloudflare handlers to send alerts on verification/enqueue.
• Extends WebhookVerificationService.handleWithQueue and queue handling to dispatch alerts after successful enqueue, and updates queue responses to include platform and eventId when queued.
• Enhances notification utilities: added robust source/eventId resolution and non-empty string picking in src/notifications/utils.ts, defaulting source to unknown when unresolved, and preventing undefined metadata in control alert payloads via withoutUndefined in src/upstash/controls.ts.
• Makes Slack and Discord payload builders respect custom title and message while falling back to updated default constants in src/notifications/constants.ts and payload builders.
• Updates README with adapter-level alert usage examples and documents behavior differences between queue and non-queue modes.
• Adds tests in src/test.ts to validate custom title/message passthrough and fallback behavior for alerts.

Testing

• Ran the project test suite with npm test, which included the two new alert tests for custom title/message passthrough and fallback behavior, and confirmed they passed.
• Exercised handler flows for both queue and non-queue paths manually via unit-style checks in src/test.ts to verify dispatch runs only when alert config is present and successful enqueue/verification occurs.

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Codex Task