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)

[anaslimem]

@benbrandt Fixed! Changed to check for any non-null, non-false value so objects {} are now detected correctly. PTAL when you get a chance.

[nikomatsakis]

Hey @anaslimem -- I didn't realize this PR was here. I think that #114 fixed the first problem you identified, right? Any chance you can rebase and we can tackle the second?

[anaslimem]

Hello @nikomatsakis I have merged main into this branch. Also i think it is not the same as #114 Because each PR fixed something