fix(airplay): carry the HTTP status code in the error instead of re-parsing the message

[nikhilshastry2003]

The follow-up #44 named as worth doing on its own: "the status code is not carried through AirPlayError::Negotiation, so there is nothing better to match on today."

post_stream turned a failed status line into the text of a Negotiation error, and run_session parsed the code back out of that text to decide whether to fall back into HAP pairing. #44 made that parse read the first line only, which fixed the header-digit false positives, but the design stayed: the message format was a contract between two functions that nothing enforced. Reword the error and the retry silently stops.

What changes

• AirPlayError::HttpStatus { request, code, status_line } is what a non-success response becomes. The code travels as a number; the status line is kept verbatim for the human reading it. The message is unchanged: POST /stream failed: HTTP/1.1 404 Not Found.
• should_retry_with_pairing(&AirPlayError) replaces wants_authentication(&str) + status_code(&str) in session.rs. It matches on code: 401 | 403 | 404 | 470 | 501 — the same five statuses fix(airplay): stop refusing receivers by model string, and widen the auth retry #44 established, with its receiver table kept in the doc comment. A Negotiation error whose text happens to quote a status line is no longer a retry, because there is no text for a number to be mis-read out of.
• One status-line parser. The crate had three: http_session::status_is_success, session::status_code, and the inline one in hap_pairing::check_http_status. http_session::status_code is now the only one; status_is_success, the new http_failure and check_http_status all go through it. Behaviour matches the two production parsers (second whitespace token of the first line).
• The encrypted POST /stream check in negotiate_with_auth used status_line.contains("200") — the substring pattern fix(airplay): stop refusing receivers by model string, and widen the auth retry #44 removed one layer up. It now goes through status_is_success and produces the same typed error.

Tests

The auth-fallback tests keep every case from #44 — the real receiver statuses (404 / 470 / 401), the original two (501 / 403), and each retryable-code-inside-a-header regression — but build the real error through http_failure, so they exercise the path production takes rather than a hand-made string.

New: a typed error carries the right code and status line; its Display reads as before; an unreadable status line stays an untyped Negotiation; a Negotiation error quoting HTTP/1.1 404 Not Found does not retry; status_code reads only the first line.

Verified

• cargo fmt --all -- --check clean
• cargo clippy -p openplay-airplay --all-targets --all-features -- -D warnings clean
• cargo test -p openplay-airplay — 71 + 27 passed (was 66 + 27)

Only openplay-airplay is touched, so unlike #47 everything here was compiled and run locally; nothing is left to CI alone.