Confidential jdk annotate

[Abhishek1152]

• Annotate String methods (substring, concat, replace, split, trim, toLowerCase, etc.) with @PolyConfidential so confidentiality propagates through string operations
• Fix Object.toString() return type from @confidential to @PolyConfidential
• Add @PolyConfidential to StringBuilder constructor, Enum.name(), Enum.valueOf(), and Pattern.matcher()