test: cover ThreadMessageSearch filtering rules

Exercise empty queries, case-insensitive body and sender matching, and
blank-body exclusion.

bitchatTests/ThreadMessageSearchTests.swift

@@ -0,0 +1,75 @@
+import BitFoundation
+import Foundation
+import Testing
+@testable import bitchat
+
+struct ThreadMessageSearchTests {
+    private func message(
+        id: String,
+        sender: String,
+        content: String
+    ) -> BitchatMessage {
+        BitchatMessage(
+            id: id,
+            sender: sender,
+            content: content,
+            timestamp: Date(),
+            isRelay: false,
+            senderPeerID: PeerID(str: "0011223344556677")
+        )
+    }
+
+    @Test("empty query returns the original timeline")
+    func emptyQueryReturnsAll() {
+        let messages = [
+            message(id: "1", sender: "alice", content: "hello mesh"),
+            message(id: "2", sender: "bob", content: "wave back")
+        ]
+
+        let result = ThreadMessageSearch.matchingMessages(in: messages, query: "   ")
+        #expect(result == messages)
+    }
+
+    @Test("matches message body case-insensitively")
+    func matchesContent() {
+        let messages = [
+            message(id: "1", sender: "alice", content: "Meet at NOON"),
+            message(id: "2", sender: "bob", content: "see you later")
+        ]
+
+        let result = ThreadMessageSearch.matchingMessages(in: messages, query: "noon")
+        #expect(result.map(\.id) == ["1"])
+    }
+
+    @Test("matches sender nickname")
+    func matchesSender() {
+        let messages = [
+            message(id: "1", sender: "alice", content: "ping"),
+            message(id: "2", sender: "mallory", content: "pong")
+        ]
+
+        let result = ThreadMessageSearch.matchingMessages(in: messages, query: "MALL")
+        #expect(result.map(\.id) == ["2"])
+    }
+
+    @Test("skips whitespace-only message bodies")
+    func skipsBlankBodies() {
+        let messages = [
+            message(id: "1", sender: "alice", content: "   "),
+            message(id: "2", sender: "bob", content: "visible")
+        ]
+
+        let result = ThreadMessageSearch.matchingMessages(in: messages, query: "alice")
+        #expect(result.isEmpty)
+    }
+
+    @Test("returns empty when nothing matches")
+    func noMatches() {
+        let messages = [
+            message(id: "1", sender: "alice", content: "hello")
+        ]
+
+        let result = ThreadMessageSearch.matchingMessages(in: messages, query: "missing")
+        #expect(result.isEmpty)
+    }
+}