Fix double escaping of non-printable bytes in SVCB string-list values

[gaoflow]

_StringList.to_text() (used by the alpn and docpath SvcParams) applies the two SVCB escaping levels in the wrong order for bytes outside 0x20-0x7e: the list-level escape renders 0x00 as \000, and dns.rdata._escapify() then escapes that backslash, emitting \\000. The parser composes the levels correctly per RFC 9460 appendix A.1 (_unescape decodes \ddd, then _split handles \, and \\), so it reads \\000 back as the three literal characters 000 — a record's own to_text() output re-parses to a different value. ALPN ids are opaque bytes on the wire, so a record received from the wire and dumped to a zone file can silently change value when reloaded:

>>> rd = dns.rdata.from_text("IN", "HTTPS", r'1 . alpn="h\0002"')
>>> rd.params[1].ids
(b'h\x002',)
>>> rd.to_text()
'1 . alpn="h\\\\0002"'
>>> dns.rdata.from_text("IN", "HTTPS", rd.to_text()).params[1].ids
(b'h0002',)

The fix makes the inner escape purely list-level (\ and ,, operating on bytes) and leaves all character-string escaping to the single outer dns.rdata._escapify(). Comma/backslash escaping in printable ids is emitted exactly as before; non-printables now come out as \ddd and a quote byte as \", both of which the existing parser already accepts.

test_misc_escape pinned the old \\010\\010 output; updated it to the round-trippable form plus an assertion that the expected text parses back equal.