Upstash REST wrong number of arguments: your path segments dont match the command arity

# Upstash REST: ERR wrong number of arguments for the command

## What you see

`{ "error": "ERR wrong number of arguments for 'get' command" }`
with HTTP 400.

## Cause

The URL path segments after the command do not match the commands
arity. REST maps path segments to arguments positionally:

- GET foo becomes /get/foo (one argument)
- /get/foo/bar is two arguments to GET: wrong.
- MGET takes many: /mget/foo1/foo2/foo3 is correct.

The usual mistake is forgetting that some commands take option flags
as extra path segments: SET with expiry is
/set/foo/bar/EX/100, not /set/foo/bar with the expiry somewhere else.

## Confirm

Count the segments. Compare against the Redis command reference for
that command. Then run the same command in redis-cli with the same
arguments to prove the arity.

## Fix

- Values containing slashes break positional parsing: a value like
  "a/b" becomes two segments. POST the value as the request body
  instead: the body is appended as the last parameter.
- For SET with options after the value, pass the options as query
  params on a POST: /set/foo?EX=100 with the value in the body.
- Or send the whole command as a JSON array body: '["SET", "foo",
  "bar", "EX", 100]'.

## Verify

The same curl returns 200 with the expected result.