Part of epic #3602.

Scope

Extend ProviderEntry with the new fields needed to declare a native gonka provider, and add the Gonka variant to ProviderKind. No provider implementation yet — that lands in #9.

Files to modify

• crates/zeph-config/src/providers.rs:
  • ProviderKind::Gonka variant; as_str() returns \"gonka\".
  • New GonkaNode { base_url: String, address: String } struct.
  • New fields on ProviderEntry (both #[serde(default)]):
    • pub gonka_nodes: Option<Vec<GonkaNode>>
    • pub gonka_chain_prefix: Option<String> (default \"gonka\")
  • Validator: when type = \"gonka\", gonka_nodes must be non-empty; otherwise return a config error.

Acceptance

• cargo nextest run -p zeph-config -- providers includes new tests:
  • Round-trip of a gonka entry with three nodes
  • Validator rejects empty gonka_nodes
  • Default chain_prefix is \"gonka\" when omitted
• Existing configs (which never set these fields) still parse correctly.
• cargo +nightly fmt --check and cargo clippy --workspace --features full -- -D warnings pass.

Depends on

None.

Size

S (~1.5h)