fix(server): merge _input_out when using standalone middlewares

[jussisaurio]

Closes #4947

🎯 Changes

There was a problem in the current implementation of experimental_standaloneMiddleware
where the input type was not merged properly in the middleware chain.

Example:

  const t = initTRPC.create();
  const schemaA = z.object({ valueA: z.string() });
  const schemaB = z.object({ valueB: z.string() });

  const mw1 = experimental_standaloneMiddleware<{
    input: z.infer<typeof schemaA>;
  }>().create((opts) => {
    return opts.next();
  });

  const mw2 = experimental_standaloneMiddleware<{
    input: z.infer<typeof schemaB>;
  }>().create((opts) => {
    return opts.next();
  });

  const combinedInputThatSatisfiesBothMiddlewares = z.object({
    valueA: z.string(),
    valueB: z.string(),
  })

  t.procedure
    .input(combinedInputThatSatisfiesBothMiddlewares)
    .use(mw1)
    .use(mw2) // <-- error: input only has { valueA: string }
    .query(
      ({
        input: { valueA, valueB }, // <-- error: input only has { valueB: string }
      }) =>
        `valueA: ${valueA}, valueB: ${valueB}`,
    );

Funnily enough, since tRPC merges multiple .input() schemas under the hood, it was
possible to never encounter this problem if you e.g. defined another .input() parser
after a standalone middleware, e.g.:

t.input(onlyA).use(requiresAMiddleware).input(otherStuff).query(opts => /* opts.input has merged onlyA and otherStuff! */

This commits fixes the problem by merging _input_out whenever necessary.

✅ Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

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[juliusmarminge]

Can you move this here

https://github.com/trpc/trpc/tree/main/packages/tests/server/regression

[KATT]

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