Routing: run all handlers sequentially and add startup validation

[MehakBindra]

Summary

• All matching routes now run sequentially for non-invoke activities. Previously only the first matching route executed, silently swallowing any subsequent handlers.
  Registration order is preserved.
• Invoke routing remains first-match-wins. Invoke activities require exactly one InvokeResponse back to Teams — running multiple handlers is undefined behaviour.
• Startup throws if OnInvoke and specific invoke handlers are mixed. Registering both OnInvoke and eg. OnAdaptiveCardAction guarantees one response is silently swallowed. The valid approaches are: OnInvoke exclusively with an internal if/else, or specific handlers exclusively.
• Startup throws if the same route is registered twice. Duplicate registration is assumed to be a bug.
• Removed ILogger from Router. The logger passed was ILogger. Startup errors now throw
  exceptions instead of logging.

[corinagum]

First: I think running all matching handlers sequentially is the right call.

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I'm leaving this as a non-blocking comment: I'm reallly hesitant about removing the middleware chain. What we're sacrificing by getting rid of next():

• stopping the chain - handler can determine whether to continue or stop the chain
• before/after logic - code executed both before and after calling next(), like Express.js middleware wrapping

[corinagum]

I'm very hesitant about this one. Will leave some comments in Teams.

[singhk97]

one nit: but looks good to me otherwise