fix(start): Fix `FormData` handling in server functions

[np-13]

• Ensure context is included when serializing FormData on the client (serialized using default transformer and added to the __TSR_CONTEXT field in the payload)
• Add input type check on the server and, if it's FormData, extract context value from the __TSR_CONTEXT field before passing the actual payload to the middleware
• Add E2E test for multipart/form-data form submission

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commit: be75338