feat: Add HTTP headers to ServerCallContext

Include HTTP headers in the ServerCallContext to provide
handlers with easy access to critical request metadata.

This change maintains consistency with the existing design
pattern of using the call context for request-specific data
and simplifies handler implementation by making headers
readily available via `context.state['headers']`.

Author: PratikMahajan

src/a2a/server/apps/jsonrpc/jsonrpc_app.py

@@ -89,6 +89,7 @@ def build(self, request: Request) -> ServerCallContext:
         with contextlib.suppress(Exception):
             user = StarletteUserProxy(request.user)
             state['auth'] = request.auth
+        state['headers'] = dict(request.headers)
         return ServerCallContext(user=user, state=state)