Fix vault example: correct hashicorp property separator and unmarshalled JDBC result

camel-hashicorp-vault resolves {{hashicorp:secret:<path>#<key>}} using
'#' to separate the secret path from the field key, not '/'. The
datasource bean used '/', so the placeholders never resolved and the
app failed to start against a real Vault. Also aligned the README's
secret name (myDatabase) with the path actually used in
cars-datasource.yaml (database).

Separately, GET /api/cars 500'd with NoTypeConversionAvailableException
converting the JDBC SELECT's ArrayList result to InputStream for the
text/plain REST response. Marshal the result to JSON (adding
camel-jackson-starter) and serve it as application/json instead.

Verified end-to-end against real Vault and Postgres containers: app
starts, GET returns [], POST inserts a row, GET then returns it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HseDjsECuZhsCXxxioiD1k