feat(ffi): add dash_spv_ffi_config_clear_peers to remove default peers

When creating an SPV config for regtest, a default peer at
127.0.0.1:19899 is added. This doesn't match dashmate's Docker
setup which maps Core P2P to different ports (e.g. 20001). FFI
consumers (iOS/Swift) couldn't clear these defaults before adding
their own peers, causing the SPV client to enter exclusive mode
with a dead peer alongside the intended one.

Adds dash_spv_ffi_config_clear_peers() so FFI consumers can remove
default peers before adding custom ones via add_peer.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: QuantumExplorer

dash-spv-ffi/src/config.rs

@@ -166,6 +166,25 @@ pub unsafe extern "C" fn dash_spv_ffi_config_add_peer(
     }
 }
 
+/// Removes all configured peers from the configuration
+///
+/// This is useful when the caller wants to start with a clean slate
+/// before adding custom peers via `dash_spv_ffi_config_add_peer`.
+///
+/// # Safety
+/// - `config` must be a valid pointer to an FFIClientConfig created by dash_spv_ffi_config_new/mainnet/testnet
+/// - The caller must ensure the config pointer remains valid for the duration of this call
+#[no_mangle]
+pub unsafe extern "C" fn dash_spv_ffi_config_clear_peers(
+    config: *mut FFIClientConfig,
+) -> i32 {
+    null_check!(config);
+
+    let cfg = unsafe { &mut *((*config).inner as *mut ClientConfig) };
+    cfg.peers.clear();
+    FFIErrorCode::Success as i32
+}
+
 /// Sets the user agent string to advertise in the P2P handshake
 ///
 /// # Safety