Fix ClientError arguments and crashes on unusable 4xx bodies

[pucedoteth]

Two problems in API._handle_exception, both in the 4xx branch.

1. The fallback raises pass arguments in the wrong positions

ClientError.__init__ is (status_code, error_code, error_message, header, error_data), but both fallbacks pass:

raise ClientError(status_code, None, response.text, None, response.headers)

so header is None and the HTTP headers land in error_data. The success path passes them correctly, so the same exception type carries headers in a different attribute depending on what the body looked like.

This is the ordinary path, not an edge case. The API answers a bad request with a plain-text body, which is not valid JSON, so every real 4xx takes the JSONDecodeError fallback:

$ curl -s -X POST -H 'Content-Type: application/json' \
    -d '{"type":"nonexistentType"}' https://api.hyperliquid-testnet.xyz/info
Failed to deserialize the JSON body into the target type      # HTTP 422

Anything reading err.header to back off on a rate limit sees None today.

2. Non-object JSON bodies raise the wrong exception type

The body is only checked for None before being used as a mapping. json.loads returns None only for the literal null — a JSON string, array or number is equally not a dict:

body	before
"Unauthorized"	AttributeError: 'str' object has no attribute 'get'
["a","b"]	AttributeError: 'list' object has no attribute 'get'
429	AttributeError: 'int' object has no attribute 'get'
{"error": "rate limited"}	KeyError: 'code'

The last row is what an intermediary with its own error schema returns — a gateway, proxy, or the local node at LOCAL_API_URL. In every case the caller gets an exception the SDK does not define, instead of the ClientError they wrapped the call to catch.

Fix

Accept a body only when it is a dict carrying both code and msg; otherwise raise a ClientError holding the raw text as the message, with the headers in header. A well-formed body is unpacked exactly as before.

Testing

Adds tests/api_test.py — the well-formed body is unpacked as before, 5xx still raises ServerError, and eight unusable bodies (including the plain-text one the live API actually returns) all produce a well-formed ClientError.

Reverting only hyperliquid/api.py and keeping the tests: 8 of 11 fail, showing both bugs —

E   AssertionError: assert None == {'x-ratelimit-remaining': '0'}   # x2, the swapped args
E   AttributeError: 'str' object has no attribute 'get'
E   AttributeError: 'list' object has no attribute 'get'
E   AttributeError: 'int' object has no attribute 'get'
E   KeyError: 'code'                                                # x3

Full suite: 47 passed. black --check, isort --check-only and mypy clean.

Doesn't overlap #300, which adds messages to the exception classes in utils/error.py and leaves this function alone.