Add bridge critical error action

[boblangley]

Summary

Adds a public BridgeConfiguration.DefineCriticalErrorAction(...) API so bridge hosts can configure a response to critical errors using the same delegate shape as regular NServiceBus endpoints.

The configured action is stored in the finalized bridge configuration and applied to the raw endpoint configuration for both bridge proxy endpoints and send-only dispatcher endpoints. This keeps the action wired through transport HostSettings.CriticalErrorAction, matching the convention used by NServiceBus Core.

Testing

• Added bridge configuration tests for null handler validation and finalized configuration storage.
• Added endpoint proxy factory tests that use a fake transport to raise critical errors through HostSettings.CriticalErrorAction for both dispatcher and proxy raw endpoints.
• Updated public API approval for the additive API change.