Review follow-up on the keyless Exa UTF-8 fix:
- `_parse_mcp_body` split SSE frames on `\n` only, so a body using bare CR
line terminators (permitted by the SSE spec, and parsed fine before via
`splitlines()`) failed with "Unrecognized MCP response shape". Split on
the SSE terminators CRLF / CR / LF instead.
- `mcp_call` decoded the body as UTF-8 unconditionally, ignoring a charset
the server did declare. Decode with the declared charset when the
Content-Type carries one and fall back to UTF-8 only when it is absent.
- The HTTP >= 400 branch and `_keenable_request` still surfaced
`response.text`, so a non-ASCII error body from a charset-less text/*
response reached the user as ISO-8859-1 mojibake. Both now go through the
same decode helper as the success path.
4b00842536