Fix two broken key-generation code paths

[ZenulAbidin]

Summary

Fixes two deterministic, reproducible bugs in the key-generation API. Both are exercised by the public surface (create_keypair is exported from zpywallet/__init__.py), both fail 100% of the time, and neither was covered by tests.

Bug 1 — PrivateKey.from_random() always returns the zero scalar

i = 0
while i < 0 or i >= secp256k1.N:   # 0 is not < 0 and not >= N, so the loop body never runs
    i = int.from_bytes(Random.new().read(32), byteorder="big")
return PrivateKey.from_int(i, network)   # from_int(0) -> coincurve rejects it

i starts at 0; the guard never rejects 0, so the loop body never executes and from_int(0) raises:

ValueError: Secret scalar must be greater than 0 and less than 115792089237316195423570985008687907852837564279074904382605163141518161494337.

Even if the loop had run, 0 would still have been accepted. Fix: reject 0 as well (while i <= 0 or i >= secp256k1.N).

Bug 2 — create_keypair() passes raw bytes to PrivateKey()

random_bytes = urandom(32)
prv = PrivateKey(random_bytes, network=network)   # __init__ calls ckey.to_hex()

PrivateKey.__init__ expects a coincurve.PrivateKey (it calls ckey.to_hex()), but raw bytes were passed, so every call raised:

AttributeError: 'bytes' object has no attribute 'to_hex'

Fix: build the key via the (now-correct) PrivateKey.from_random(network), which also guarantees a valid in-range scalar.

Reproduction (before)

[FAIL] create_keypair(BTC) AttributeError: 'bytes' object has no attribute 'to_hex'
[FAIL] create_keypair(ETH) AttributeError: 'bytes' object has no attribute 'to_hex'
[FAIL] PrivateKey.from_random ValueError: Secret scalar must be greater than 0 ...

Tests

• tests/test_12_keys.py::test_008_from_random_returns_valid_scalar — asserts from_random yields a scalar in [1, N-1] and a usable address, across repeated draws.
• tests/test_05_zpywallet.py::test_010_create_keypair — asserts create_keypair returns a consistent (PrivateKey, PublicKey) pair for BTC (base58 address) and ETH (0x hex address).

Validation

Check	Result
Full suite (pytest tests)	166 passed, 7 skipped (was 164 + 2 new)
flake8 (repo config)	Passed
Repro probe, post-fix, 5x	create_keypair + from_random all OK

No behavioral change to any already-working path; the diff to library code is 2 lines.