Confidential checker toString and generics fixes

[Abhishek1152]

Changes:

• Changed Object.toString() return type from @confidential to @PolyConfidential in the annotated JDK (typetools/jdk), so the return type follows the receiver's confidentiality. This allowed removing the toString() special-case logic in ConfidentialAnnotatedTypeFactory that was manually handling the return type override.
• Removed UPPER_BOUND from @NonConfidential default qualifier to support generics with @confidential type arguments.
• Added test cases for method arguments, hard-coded literals, arithmetic, casting, control flow, and generics behavior.