GroupV2 should own its network config instead of injecting de_mls::ConversationConfig across layers

[osmaczko]

Follow-up from PR #167 review. GroupV2 timing/policy is currently a bare re-export of de_mls::ConversationConfig (aliased GroupV2Config) threaded through several layers:

• Core::set_group_v2_config / ServiceContext.group_v2_config (core/conversations/src/core.rs, service_context.rs)
• the for_test ServiceContext scaffolding (core/conversations/src/service_context.rs, test-only)
• the client builder's group_v2 option (crates/client/src/builder.rs)

This leaks the de_mls type from core through libchat to the client builder, crossing abstraction layers and risking interop churn as de_mls evolves. It is Option/None-defaulted behind the builder, so current developer impact is small.

What's needed

• GroupV2 provides a sensible default configuration for network operation; the millisecond test profile stays in the test harnesses.
• Remove the cross-layer config plumbing (a client-owned config mapped to de_mls internally, or dropped once de_mls owns sane defaults). Relates to feat!: caller-owned WallClock time source vacp2p/de-mls#130.

Raised by @jazzz (review comments on builder.rs, service_context.rs, core.rs set_group_v2_config, and the approval note).