fix(mcp): resolve all MCP spec compliance issues

[giveen]

Description of Changes

• Replace manual stdio transport with SDK CommandTransport for proper SIGTERM → wait → SIGKILL process lifecycle
• Reuse single SDK client instance across all server sessions
• Wire ToolListChangedHandler to re-discover tools on dynamic changes
• Handle all MCP content types (AudioContent, ResourceLink, EmbeddedResource) plus default warning for unknown types
• Add SSE and Streamable HTTP transport support for remote servers
• Capture stderr from stdio subprocesses on connection failure
• Log warning on config file write failure instead of silent discard
• Suppress unnecessary 'roots' capability advertisement

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[mlhher]

@giveen

Great one thanks!

I evaluated the code a bit and noticed two things that might cause issues, please take a look at them:

• handleToolListChanged modifies the c.tools map asynchronously, so we need a sync.RWMutex on the Client struct to prevent panics.
• stderrBuf races because a background routine writes to it while ConnectFromConfig reads it on failure, so we should wrap the buffer with a mutex.

Also a minor thing but would be good to have a tiny time.Sleep before checking the buffer, since an instant failure might read it before the bg routine even writes the error.

Will try this out a little more afterwards and if nothing new pops up it will get merged, thanks!

[giveen]

@mlhher