Replace HTTP status string parsing with structured errors

[Faerkeren]

Problem

REST error handling depends on parsing exception message text.

src/haclient/infra/rest_aiohttp.py:131-133 converts every non-401 HTTP error into a generic HAClientError string. get_state() then detects missing entities by checking whether "HTTP 404" appears in str(err) at src/haclient/infra/rest_aiohttp.py:186-190.

That is brittle. If the message format changes, or another error body happens to include that text, get_state() behavior changes.

Suggested Fix

Introduce a structured HTTP exception, for example HTTPError(status: int, method: str, path: str, body: str), derived from HAClientError.

Then:

• Raise HTTPError from _request() for HTTP failures.
• Check err.status == 404 in get_state().
• Keep authentication mapped to AuthenticationError.
• Add tests that get_state() returns None only for real 404 responses.

Rationale

Status codes are structured data. Encoding them into strings and parsing them back out is avoidable fragility in a reliability-sensitive client.