Problem

When developers use a string literal directly in a raw constraint, they encounter a confusing compiler error that doesn't clearly indicate the actual problem:

#[account(constraint = "my string")]

This produces:

error[E0600]: cannot apply unary operator `!` to type `&'static str`
  --> programs/ido-pool/src/lib.rs:284:10

This error message originates deep in the derive macro and doesn't help developers understand what they did wrong.

Solution

This PR adds early validation in the constraint parser to detect string literals and provide a clear, actionable error message:

error: constraint value must be a boolean expression, not a string literal.
       Help: Raw constraints expect expressions that evaluate to boolean values.
       If you need to compare a field to a string, use: constraint = my_field == "my string"

Changes

• Added string literal detection in lang/syn/src/parser/accounts/constraints.rs
• Returns an Anchor-specific ParseError before the expression reaches Rust's type checker
• Provides helpful guidance on correct usage with an example

Closes #468