ChatClient migration

[jazzz]

This PR continue preparing Cli-Client for group chats. Specifically It introduces a Builder pattern which allows clients to be constructed with both testing and production components.

Relating to De-MLS, this PR:

• Allow Clients to accept any IdentityProvider to support DelegateSigners.
• Removes reliance on legacy settings.
• Chooses a default implementation for each service.

Summary

• Introduces ChatClientBuilder, a typestate builder for constructing ChatClient that makes all four configuration slots (identity, transport, registration, storage) explicit at the type level, with sensible defaults for any unset field
• Promotes ChatClient from a 2-param <T, R> type to a fully generic <I, T, R, S> type, making the identity provider and storage backend first-class type parameters; removes the old open / open_with_registry constructors in favour of the builder
• Adds storage_config() on the builder as a convenience that accepts a StorageConfig and constructs ChatStorage inline, so callers don't need to import ChatStorage directly
• Fixes re-entrant message filtering in GroupV1Convo: echoed outbound messages (including MLS Commits from add_member) are now suppressed via a bounded hash cache (OUTBOUND_HASH_CACHE_SIZE = 25) to prevent the sender from re-processing its own messages on loopback transports
• Re-exports ChatStorage, ChatStore, and IdentityProvider from logos-chat so downstream crates don't need to reach into internal dependencies
• Updates chat-cli (app.rs, ui.rs, main.rs) and the message-exchange example to use the new 4-param ChatApp<I, T, R, S> and builder API

[osmaczko]

I like the builder pattern 👷‍♂️