refactor(auth): use SecureRandom for random hex string generation

[carloea2]

What changes were proposed in this PR?

This PR updates random hex string generation in the Auth module to use SecureRandom instead of Random.

It also changes the hex string construction to use fixed-width integer-to-hex conversion (%08x), so each generated integer consistently contributes 8 hexadecimal characters. This keeps the generated value length stable and makes the implementation more predictable across environments.

Current Implementation:

texera/common/config/src/main/scala/org/apache/texera/config/AuthConfig.scala

Lines 48 to 55 in 3e46cea

	private def getRandomHexString: String = {
	val bytes = 32
	val r = new Random()
	val sb = new StringBuffer
	while (sb.length < bytes)
	sb.append(Integer.toHexString(r.nextInt()))
	sb.toString.substring(0, bytes)
	}

Any related issues, documentation, discussions?

Closes #4284

How was this PR tested?

Manually verified that the updated implementation:

• returns a 32-character hexadecimal string
• preserves the existing method behavior and output length
• uses fixed-width 8-character hex chunks for each generated integer

Was this PR authored or co-authored using generative AI tooling?

No

[bobbai00]

LGTM!