Add builder for push queues

[leynos]

Summary

• add PushQueuesBuilder with configuration setters
• deprecate bounded* constructors and expose PushQueues::builder
• update tests to use the new builder

Testing

• make fmt
• make lint
• make test

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https://chatgpt.com/codex/tasks/task_e_68aeba8d630c8322bcab55de80b2ef06

Summary by Sourcery

Introduce a builder pattern for PushQueues to configure capacities, rate limiting, and an optional dead-letter queue; deprecate existing bounded constructors in favor of the builder and update tests accordingly.

New Features:

• Add PushQueuesBuilder with methods for high_capacity, low_capacity, rate, dlq, and build

Enhancements:

• Add PushQueues::builder to initialize the builder using default settings
• Deprecate bounded, bounded_no_rate_limit, bounded_with_rate, and bounded_with_rate_dlq constructors in favor of the builder API

Documentation:

• Update documentation comments to reference PushQueuesBuilder and its methods

Tests:

• Refactor existing tests to use the new builder API instead of deprecated constructors