Add e2e config startup tests

Test server startup with various config settings: optional fields,
log levels, TLS hosts, LSPS2 variations, and bitcoind RPC with
localhost hostname. Add negative tests for invalid configs ensuring
proper error messages. Dedicated chain source tests verify each
backend explicitly.

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

Author: benthecarman

e2e-tests/src/lib.rs

@@ -354,6 +354,26 @@ fn spawn_server(
 	(child, params)
 }
 
+/// Start ldk-server with the given config and expect it to fail (exit non-zero).
+/// Returns the stderr output for assertion in tests.
+pub fn start_expect_failure(
+	bitcoind: &TestBitcoind, config_fn: impl FnOnce(&TestServerParams) -> String,
+) -> String {
+	let (child, ..) = spawn_server(bitcoind, config_fn);
+
+	let output = child
+		.wait_with_output()
+		.unwrap_or_else(|e| panic!("Failed to wait for ldk-server process: {}", e));
+
+	assert!(
+		!output.status.success(),
+		"Expected server to fail but it exited with status: {}",
+		output.status
+	);
+
+	String::from_utf8_lossy(&output.stderr).to_string()
+}
+
 /// Find an available TCP port by binding to port 0.
 pub fn find_available_port() -> u16 {
 	let listener = TcpListener::bind("127.0.0.1:0").unwrap();