minor: reduce unnecessary string allocations

[andygrove]

Summary

Replace string concatenation patterns with more efficient alternatives.

Changes:

1. Tokenizer - Replace String::from() + &str with format!():

   // Before
   Token::Placeholder(String::from("?") + &s)
   Token::Placeholder(String::from("$") + &value)
   
   // After
   Token::Placeholder(format!("?{s}"))
   Token::Placeholder(format!("${value}"))

2. Parser - Replace placeholder value concatenation:

   // Before
   tok.to_string() + &ident.value
   
   // After
   format!("{tok}{}", ident.value)

3. AST Display - Replace string concatenation in FLUSH statement:

   // Before
   " ".to_string() + &display_comma_separated(tables).to_string()
   
   // After
   format!(" {}", display_comma_separated(tables))

4. Parser COPY TSV - Avoid unnecessary clone:

   // Before
   let mut content = String::from("");
   values.push(Some(content.to_string()));
   content.clear();
   
   // After
   let mut content = String::new();
   values.push(Some(std::mem::take(&mut content)));

The format!() macro calculates required capacity upfront and allocates once. std::mem::take() moves the String instead of cloning it.

Test plan

• All existing tests pass
• cargo clippy passes

🤖 Generated with Claude Code

[iffyio]

LGTM! Thanks @andygrove!