fix(credentials): validate user and key fields when parsing credentials file

[r2dedios]

Summary

• Validate that user and key fields are not empty when parsing the credentials file in ReadCloudAccounts
• Accounts with missing credentials are skipped and reported via ErrMissingCredentials
• Scanner and Agent callers log a warning for skipped accounts but continue with valid ones
• If no valid accounts remain, the error propagates as before (fatal)

Test plan

• make go-unit-tests — all pass, 100% coverage on credentials
• make lint — 0 issues
• make local-build — compiles clean
• Tests cover: missing user, missing key, mixed valid/invalid accounts

Closes #215