fix(cipher/diffiehellman): use valid random private keys in key-exchange test

[rajpreetcodes]

Description of Change

The Diffie-Hellman key-exchange test produced its "random" private keys by calling rsa.GenerateKey(rand.Reader, 31) and reading .D.Int64().

Go 1.24 introduced a minimum RSA key size of 1024 bits, so rsa.GenerateKey with a 31-bit size now returns the error rsa: key too small and a nil key. The test ignored that error (alicePrvKey, _ := ...) and then dereferenced the nil key (alicePrvKey.D.Int64()), producing a nil-pointer panic:

--- FAIL: TestDiffieHellmanKeyExchange/Test_2:_Key_Exchange
panic: runtime error: invalid memory address or nil pointer dereference
    .../diffiehellmankeyexchange_test.go:28

Because the CI workflow uses setup-go with go-version: '^1.18' (which resolves to the latest Go release), this breaks the build on Go ≥ 1.24.

This change generates the private keys directly with crypto/rand in the valid Diffie-Hellman range [1, primeNumber-1], removing the RSA dependency. It also makes the test more meaningful: the old .D.Int64() value could be negative, which sent modularExponentiation down a degenerate path where the equality assertion held even for invalid inputs.

Verification:

• go test ./cipher/diffiehellman/ passes (ran with -count=20, not flaky)
• go test ./... is green
• go vet ./... clean
• gofmt clean

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - GoDoc guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Fixes a nil-pointer panic in the Diffie-Hellman key-exchange test on Go ≥ 1.24 (rsa: key too small).

[codacy-production]

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