Improve RSA exponent checks

[picatz]

This pull request enhances RSA key validation in the pkg/jwk package by introducing stricter checks for RSA modulus size and public exponent values, along with comprehensive tests to ensure the robustness of these validations. The key changes include updates to the RSAPublicKey function and the addition of new test cases.

RSA Key Validation Enhancements:

• Minimum RSA Modulus Size Enforcement: Updated RSAPublicKey to reject RSA keys with a modulus size smaller than 2048 bits. This ensures compliance with security best practices.
• Public Exponent Validation: Added checks in RSAPublicKey to ensure the public exponent is a positive integer between 2 and math.MaxInt32, rejecting invalid values.

Test Coverage Improvements:

• Test for Large Exponent: Added a test case in TestErrorMessages to verify that excessively large public exponents are correctly rejected.
• Test for Modulus Size Validation: Introduced TestRSAModulusSizeValidation to ensure RSA keys with moduli smaller than 2048 bits are rejected, while valid moduli are accepted.
• Test for Public Exponent Validation: Added TestRSAPublicKeyExponentValidation to test various exponent values, ensuring invalid exponents are rejected and valid ones are accepted.

These changes improve the security and reliability of RSA key handling in the pkg/jwk package.

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https://chatgpt.com/codex/tasks/task_e_6866d2118ff0833195a7b340a95ffc90

[Copilot]

Pull Request Overview

This PR strengthens RSA key validation by enforcing a minimum modulus size and stricter exponent checks, and adds corresponding tests.

• Enforce a 2048-bit minimum RSA modulus in RSAPublicKey
• Validate public exponent is between 2 and math.MaxInt32
• Add tests for large exponents, small moduli, and various exponent edge cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/jwk/jwk.go	Added modulus bit-length check and exponent range/bit-length validation
pkg/jwk/jwk_test.go	Introduced tests for modulus size and exponent validation, and expanded error-message coverage

Comments suppressed due to low confidence (1)

pkg/jwk/jwk.go:308

• [nitpick] The error messages for invalid public exponent share the same prefix but represent different failures; consider using more specific messages (e.g., 'public exponent bit length too large' vs 'public exponent out of allowed range') to improve diagnostic clarity.

	if !e.IsInt64() || e.BitLen() > 31 {