Validate root CA certificates on startup

[vlasebian]

Summary

Validate that certificates configured as root CAs are actually CA certificates at startup, preventing misconfiguration where a server leaf certificate is accidentally set as a root CA.

Changes

• Add parseAndValidateCACerts function that validates PEM-encoded root CA certificates have the IsCA flag set.
• Reject non-CA (leaf) certificates, invalid PEM data, and non-CERTIFICATE PEM blocks with clear error messages.
• Use project error definitions (errors.DefineInvalidArgument) for consistent error handling.
• Handle trailing whitespace in PEM files gracefully.

Testing

Steps

Run the unit tests:

go test ./pkg/config/tlsconfig/... -v -run TestApplyTLSClientConfig

Results

Unit tests pass.

Regressions

• Client.ApplyTo now validates certificates before adding them to the root CA pool. Previously, any PEM certificate (including leaf certs) was silently accepted. Existing valid configurations are unaffected.

Notes for Reviewers

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Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.