test: add config persistence roundtrip tests for ConfigValue preservation (drasi-project#89)

Add comprehensive tests verifying that ConfigValue envelopes (secrets,
environment variables), camelCase property keys, and passwords survive
the full save → load YAML persistence cycle. These tests guard against
regressions where the persistence layer flattens structured config
values to resolved plaintext.

Covers: secret references, env var references (with/without defaults),
camelCase key preservation, mixed config values, password preservation,
multiple components, and server-level ConfigValue env vars.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danielgerlag