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/conformance.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: Start everything-server
         run: |
           go run ./conformance/everything-server/main.go -http=":3001" &
@@ -45,7 +45,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: "Run conformance tests" 
         uses: modelcontextprotocol/conformance@a2855b03582a6c0b31065ad4d9af248316ce61a3 # v0.1.15
         with: