fix ruff on main; add typed XenAPIError with the structured error

[acefei]

Two independent fixes, smallest first.

1. fix(ci) — the ruff job is red on main

Since examples/xscert.py landed, ruff@0.16.2 check . fails:

examples/xscert.py:29:1   I001 Import block is un-sorted or un-formatted
examples/xscert.py:262:13 B007 Loop control variable `ref` not used within loop body

Imports sorted; the loop now iterates .values() since the key is unused.
Only the ruff job was failing — build and all six import jobs were green.

2. feat(python) — keep the structured error

_call and login_with_password rendered the JSON-RPC error into a string and
discarded the object, so a caller had to substring-match to classify a failure:

except RuntimeError as e:
    if "RBAC_PERMISSION_DENIED" in str(e):   # sniffing the rendered text

XenAPIError preserves it:

except XenAPIError as e:
    if e.code == "RBAC_PERMISSION_DENIED":

attribute	meaning
.code	XAPI's error name (JSON-RPC carries it in message)
.params	XAPI's error parameters (data)
.error	the raw error object
.method	the failing call

It subclasses RuntimeError, so existing except RuntimeError keeps working,
and a non-dict error degrades to code=None instead of raising.

Version bumped to 1.0.6 so this and the earlier 3.9 annotation fix can reach
PyPI — the published 1.0.5 still declares requires-python >=3.12 yet cannot be
imported on 3.12/3.13.

Verification

Ran the three CI jobs locally against this branch before pushing:

• uvx ruff@0.16.2 check . — All checks passed
• uv venv --python 3.9 && uv pip install . && import — OK (also checked 3.14)
• uv build — async_xenapi-1.0.6.tar.gz + .whl

Motivation

Found while building a client-certificate auth demo against a real XS9 pool. The
demo currently subclasses the session to inject an SSL context and matches error
strings; with ssl_context=, session_ref and now XenAPIError, all three
workarounds go away.