fix: propagate client connection errors and check capability value truthiness

[anaslimem]

Two bugs fixed :

1. Connection errors swallowed (mcp_server/builder.rs)

   • McpServerConnection::connect_to was dropping the client connection result and always returning Ok(())
   • Now properly propagates connection failures so the caller knows the connection failed

2. Capability checks incorrect (capabilities.rs)

   • has_meta_capability only checked key presence, returning true even for explicit false or null values
   • Now checks for actual boolean true value via as_bool().unwrap_or(false)