Validate `Host` and `Origin` independently in `DnsRebindingProtectionMiddleware`

[ausi]

The current DnsRebindingProtectionMiddleware handles Host and Origin as alternatives:

• If an Origin header is present, only its hostname is checked.
• Otherwise, the Host header is checked.
• Both values are checked against the same allowedHosts list.

This causes several issues:

• An invalid Host header is not rejected when an allowed Origin header is present.
• Host and Origin represent different parties: Host identifies the target MCP server, while Origin identifies the web origin initiating the request. They commonly have different values and therefore require separate allowlists.
• Origin validation is reduced to the hostname. This makes it impossible to distinguish origins by scheme or port, even though those are part of the web-origin tuple.

Would it make sense to either:

1. Split this into separate Host and Origin validation middleware; or
2. Extend DnsRebindingProtectionMiddleware with separate allowedHosts and allowedOrigins options, validating Host on every request and additionally validating Origin whenever it is present?

I would be happy to submit a pull request if this direction is acceptable.