mcp: verify 'Origin' and 'Content-Type' headers (#842)

This PR introduces protection against cross-origin requests by
installing `http.CrossOriginProtection`.
By default, the zero value is used which does not contain any trusted
origins or bypass patterns.
The protection can be customized by providing a custom
`http.CrossOriginProtection` in `mcp.StreamableHTTPOptions`. Currently,
the deny handler set on the protection is ignored.
This default has an MCPGODEBUG option (`disableoriginverification`) to
disable the default protection.
It will be available until `v1.6.0` of the SDK.
We also increase the Go version required by the SDK to 1.25, since this
is the version that introduced `http.CrossOriginProtection`. This is in
line with the SDK's policy of supporting two newest Go versions (1.26
was released in February 2026).

Additionally, we start validating if the `Content-Type` header for
`POST` requests is set to `application/json` to avoid accepting
CORS-safelisted requests.

.github/workflows/nightly.yml

@@ -27,7 +27,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
       with:
-        go-version: "^1.25"
+        go-version: "^1.26"
     - name: Set up Node.js
       uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
       with: