fix(python): resolve HTTP client and trade message bugs

[manassehnnad1]

Summary

this PR fixes four bugs in the Python HTTP client that collectively prevent
any order from being placed successfully using the SDK as shipped.

Bug 1 — public_key called as a function (http_client.py)

TransactionSigner.public_key is a plain string attribute, not a callable.
Calling it with () raises TypeError: 'str' object is not callable
immediately, making place_orders() completely unusable.

# before
account = self.signer.public_key()

# after
account = self.signer.public_key

Bug 2 — Nonce sent as string with wrong source (http_client.py)

Two problems on the same line:

1. The nonce is wrapped in an f-string, sending it as "1778724349688225000"
   instead of the integer 1778724349688225000.
2. order_objects[0].nonce references .nonce on a plain dict returned by
   tx.to_api(). Dicts have no .nonce attribute, raising
   AttributeError: 'dict' object has no attribute 'nonce'.

The nonce variable is already correctly defined earlier in the function
and should be used directly.

# before
"nonce": f"{order_objects[0].nonce}",

# after
"nonce": nonce,

Bug 3 — px and sz sent as strings in LimitOrder.to_api() (trade.py)

Price and size are wrapped in f-strings, sending them as JSON strings
("79.6") instead of JSON numbers (79.6). The API requires numeric
values and rejects string values with a 422 error.

The same issue affects sz in MarketOrder, Stop, TakeProfit, and
TrailingStop and has been fixed in all locations.

# before
'px': f"{self.price}",
'sz': f"{self.size}",

# after
'px': float(self.price),
'sz': float(self.size),

Bug 4 — Wrong response parsing path in OrderResponse.from_api() (trade.py)

The method navigates data.payload.response.data.statuses which does not
exist in the actual API response. The real structure is
response.data.statuses. This causes from_api() to always return an
empty list [] regardless of the actual server response, making it
impossible to know if an order succeeded, failed, or what its order ID is.

# before
rlist = data.get("data",{}).get("payload",{}).get("response",{}).get("data",{}).get("statuses", [])

# after
rlist = data.get("response",{}).get("data",{}).get("statuses", [])

Impact

Bugs 1–3 together mean no order can ever be placed using the Python SDK
as shipped. Bug 4 means even if an order did go through, the caller would
never know because responses always parse as empty.

These bugs were discovered while building a Claude-powered autonomous
trading agent on BULK testnet using the Python SDK.

Testing

All fixes have been verified against the staging environment
(staging-api.bulk.trade). Orders now place successfully and responses
parse correctly, returning the expected resting status and order IDs.