Add track_delivery_metric for /api/v1/metrics endpoint

[hownowstephen]

Summary

• Adds track_delivery_metric method to Client for reporting delivery metrics via the /api/v1/metrics endpoint, replacing the deprecated /push/events endpoint
• Supports 8 metric types: opened, clicked, converted, delivered, bounced, deferred, dropped, spammed
• Adds DELIVERY_* constants and VALID_DELIVERY_METRICS to Customerio::Client
• PUSH_OPENED, PUSH_CONVERTED, PUSH_DELIVERED preserved as deprecated aliases pointing to DELIVERY_* equivalents

Based on #112 by @eserra, reimplemented with fixes for:

• valid_url? bug (passed boolean instead of URL string)
• .present? usage (Rails-only, unavailable in plain Ruby)
• Inconsistent attribute handling (now uses existing symbolize_keys/slice pattern)

Test plan

• CI passes (rspec + rubocop)
• Verify track_delivery_metric sends correct POST to /api/v1/metrics
• Verify validation: invalid metric names, missing delivery_id, whitespace delivery_id
• Verify optional attributes (timestamp, recipient, reason, href) included only when present
• Verify unknown attributes are filtered out

---

Note

Low Risk
Mostly additive client API and tests; existing push-event constants are preserved as deprecated aliases and still map to the same metric strings, keeping compatibility risk low.

Overview
Adds Client#track_delivery_metric to report delivery metrics to the new /api/v1/metrics endpoint, including input validation, attribute filtering, and support for optional fields (timestamp, recipient, reason, href).

Introduces DELIVERY_* constants plus VALID_DELIVERY_METRICS, deprecates the old PUSH_* constants as aliases, and updates docs/changelog plus a comprehensive RSpec suite for the new endpoint. Also loosens RuboCop Metrics/ClassLength to accommodate the added client code.

^{Reviewed by Cursor Bugbot for commit a1aecda. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 2fd7095. Configure here.}

[eserra]

yeah!