KAFKA-20964: Do not echo salt or saltedPassword in ScramParser errors

[shantanushukla]

JIRA: https://issues.apache.org/jira/browse/KAFKA-20964

kafka-storage format --add-scram parses SCRAM credentials in ScramParser. On Base64 decode failure, PerMechanismData built a FormatterException whose message included the user-supplied saltedpassword (and the same for salt). StorageTool prints getMessage() to stderr, so credential-adjacent input was echoed in the CLI error.

Fix

Use a generic decode-failure message and keep the IllegalArgumentException as the cause. Do not concatenate the user-supplied value.

Failed to decode given saltedPassword
Failed to decode given salt

This matches the existing request-side practice of not printing salt or salted password (AlterUserScramCredentialsRequest). Decode details remain on the cause if it is logged.

Why existing tests missed this

ScramParserTest covered valid Base64 saltedpassword/salt and missing-field errors (name, password, salt required with saltedpassword, extra keys). None of those paths throw on Base64 failure, so the decode-error message was never asserted.

Tests

These failed on trunk (message included the raw input) and pass after this change:

• testParsePerMechanismDataFailsWithInvalidSaltedPasswordBase64 — valid salt so construction reaches the salted-password decoder; saltedpassword="not-valid-base64!!!". Trunk: Failed to decode given saltedPassword: not-valid-base64!!!. After: Failed to decode given saltedPassword.
• testParsePerMechanismDataFailsWithInvalidSaltBase64 — salt="%%%". Trunk: Failed to decode given salt: %%%. After: Failed to decode given salt.

Related existing tests still pass: full ScramParserTest (including valid saltedpassword parse and toRecord()).

./gradlew :metadata:test --tests org.apache.kafka.metadata.storage.ScramParserTest :metadata:checkstyleMain :metadata:checkstyleTest :metadata:spotlessCheck

BUILD SUCCESSFUL.

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