improve context management, error handling, and replace var with let/const

[mrnazu]

This pull request contains two key updates:

1. secp256k1 JNI integration:

   • i added context destruction function named secp256k1_destroy_context to free resources after use, preventing memory leaks, which i thought it will be good contribution :)
   • implemented error handling: throws a RuntimeException in Java if context creation fails.
   • i also increased portability by using intptr_t for pointer casting to ensure compatibility across different architectures.

2. Refactoring makeTest:

   • i replaced all instances of var with const and let to align the code with modern JavaScript standards for improved scope safety and readability. No functional changes were made.

Changes:

• secp256k1 JNI:

  • added context destruction function (secp256k1_destroy_context) to free resources after use.
  • implemented error handling to throw a RuntimeException if context creation fails.
  • replaced uintptr_t with intptr_t for pointer-to-integer casting for better portability.

• makeTest refactor:

  • replaced var with const for variables that remain constant.
  • replaced var with let for variables that are reassigned during execution.

Tags:

• Type: feat, refactor
• Scope: core, secp256k1, makeTest

Checklist:

• [x] added context destruction function secp256k1_destroy_context to free resources after use.
• [x] implemented error handling for failed context creation, throwing a RuntimeException in Java.
• [x] used intptr_t for pointer casting to ensure compatibility across different architectures.
• [x] replaced var with let and const for better scoping and modern js best practices.
• [x] tested to ensure no breaking changes.
• [x] doc remains accurate and is unaffected.

[mrnazu]

@s1na , could you please review this PR and let me know if everything looks good? Thank you!

[fjl]

We do not use the Java binding of libsecp256k1, so this change is not useful to us.