feat(js/plugins/mcp): Refactor to a client manager structure, with dynamic tool calling.

[ssbushi]

• Modified the current McpClient class to a Client manager class
• GenkitMcpClient is now a dedicated single client-server connection.
• Tools are now fetched dynamically to support adding/removing/disabling clients in the manager.

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[mbleigh]

Oh also I think we should put this work into a launch branch - I created launch/mcp-redux and made #2896 to squash my own work and create a chain of commits. Can you rebase onto the launch/mcp-redux as your merge target?

[ssbushi]

One more high-level piece of food for thought: in the client manager context, I think we probably want to be very robust to misconfiguration errors. It's possible people will use this for dynamic, user-configured tools and when that happens we should recover gracefully as best we can.

So maybe in the client manager there should be an {error?: {message, status, details}} and if there's an error trying to connect to the server it is put in a disabled state, an error-level log is emitted, and the error is also saved with the server.

I'll work on this in a new commit, updating the PR to reflect the changes so far.