Fix formatting-only edits not rendering in the timeline

Components of a formatted body were identified by their plain characters, so an edit
that only added formatting (e.g. wrapping a word in inline code) kept the same ID.
SwiftUI then reused the existing MessageText, whose @State string ignores new input,
and the old unformatted text kept rendering.

Identify components by the hash of their attributed string so formatting changes
produce a new ID.

UnitTests/Sources/AttributedStringBuilderTests.swift

@@ -257,6 +257,15 @@ struct AttributedStringBuilderTests {
         #expect(components[1].attributedString.string == "Hidden content")
     }
     
+    @Test
+    func componentIDChangesWithFormatting() throws {
+        let plain = try #require(attributedStringBuilder.fromHTML("test test test"), "Could not build the attributed string")
+        let formatted = try #require(attributedStringBuilder.fromHTML("test <code>test</code> test"), "Could not build the attributed string")
+        
+        #expect(plain.string == formatted.string)
+        #expect(plain.formattedComponents.map(\.id) != formatted.formattedComponents.map(\.id))
+    }
+    
     @Test
     func detailsWithoutSummary() throws {
         let htmlString = "<details><p>Content</p></details>"