Better error handling for cache connections

[javuto]

Summary

• Added explicit mapstructure tags for Redis YAML config fields so Viper decoding is pinned to the documented YAML keys.
• Switched API/TLS YAML config loading to use isolated Viper instances instead of the package-global Viper state.
• Improved Redis connection errors for the common mismatch where osctrl sends a password but Redis has no password configured.
• Added regression coverage for Redis YAML loading in both osctrl-api and osctrl-tls.
• Added unit coverage for the Redis auth-mismatch error wrapper.

Notes

The original Redis error indicates the service is sending an AUTH password, but the Redis server is not configured with one. Operators should either clear redis.password / credentials in redis.connectionString, or configure Redis with the matching password.

Validation

• git diff --check

Could not run gofmt or go test in this sandbox because the host Go toolchain was blocked with operation not permitted.