feat(errors): improve error messages with example values

Add token type examples to make error messages more helpful. Created an
ExpectedToken enum to replace string literals for better type safety,
added example values for each token type, and enhanced error display to
show concrete examples of valid syntax.

src/command.rs

@@ -47,10 +47,47 @@ impl Command {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ExpectedToken {
+    Integer,
+    String,
+    Semicolon,
+    Statement,
+    MetaCommand,
+    EndOfFile,
+}
+
+impl ExpectedToken {
+    /// Returns an example value for this token type
+    pub fn example(&self) -> &'static str {
+        match self {
+            ExpectedToken::Integer => "42",
+            ExpectedToken::String => "\"example\"",
+            ExpectedToken::Semicolon => ";",
+            ExpectedToken::Statement => "select",
+            ExpectedToken::MetaCommand => ".exit",
+            ExpectedToken::EndOfFile => "<end of input>",
+        }
+    }
+}
+
+impl std::fmt::Display for ExpectedToken {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ExpectedToken::Integer => write!(f, "integer"),
+            ExpectedToken::String => write!(f, "string"),
+            ExpectedToken::Semicolon => write!(f, "semicolon"),
+            ExpectedToken::Statement => write!(f, "statement"),
+            ExpectedToken::MetaCommand => write!(f, "meta command"),
+            ExpectedToken::EndOfFile => write!(f, "end of file"),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub enum CommandParseError {
     Scan(ScanError),
-    UnexpectedToken(Token, &'static [&'static str]),
+    UnexpectedToken(Token, &'static [ExpectedToken]),
 }
 
 impl From<MetaCommand> for Command {