Extract Channel Provider Construction into a Factory

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Description

Design: ChannelProviderFactory trait

Added a ChannelProviderFactory trait in src/channel/mod.rs with a single create() associated function. Each provider implements it to encapsulate:

1. Provider-specific config field validation (e.g. WhatsApp's phone_number_id, Slack's app_token)
2. Temporary provider construction with a dummy SecurityGate
3. resolve_users() call
4. Final provider construction with the resolved gate and effective output config

Checklist

• Tests added or updated and passing
• cargo clippy -- -D warnings passes
• cargo fmt --check passes
• Documentation updated (if applicable)
• Linked to issue (Extract Channel Provider Construction into a Factory #7)