Fix mod update checks failing on non-JSON responses (#931)

The mod update and Find Mods feeds feed the raw HTTP body straight to
Json.decode. When the endpoint hands back something that is not JSON
(an HTML error page, a proxy/captive prompt, or a plain-text outage
message like "Exceeded secondary rate limit" -- usually still HTTP 200),
the decoder's "unexpected character 'E'" assert escaped through the
pcall and became the error message, blaming the parser instead of the
response.

Add Json.describeUnexpected() as a pre-decode content-type guard: it
returns nil for body shapes the endpoints actually publish (JSON object
or array) and otherwise a short message naming what the server sent
(HTML page / plain text / empty, with a preview). Wire it into
ModUpdate.parseReleases and ModIndex.parse, so both the sync and async
update-check paths surface the real answer instead of the parse error.
HTTP status was already checked upstream by HostShell.httpGet (non-2xx
becomes "HTTP <code> from <url> (...)"); this closes the remaining
"2xx but not JSON" gap everywhere, including bridge platforms that
expose no status or headers.

Add regression tests for plain-text, HTML, and empty bodies; strengthen
the ModIndex HTML soft-fail test.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/mods/ModIndex.lua

@@ -191,8 +191,10 @@ end
 -- Never throws: a truncated download, an HTML error page, or a feed from a
 -- future schema all come back as a message the panel can print.
 function ModIndex.parse(jsonText, Json)
+  Json = Json or require("src.link.Json")
+  local notJson = Json.describeUnexpected(jsonText)
+  if notJson then return nil, notJson end
   local ok, result, err = pcall(function()
-    Json = Json or require("src.link.Json")
     local doc, decodeErr = Json.decode(jsonText)
     if type(doc) ~= "table" then
       return nil, decodeErr or "index.json is not an object"