fix(requests): quote only the received body in status errors

[memclutter]

What

Both status-mismatch branches in Exec() built their error as
res.Status + string(body[:50]) and passed the result as the format argument
to fmt.Errorf. They now use a constant format string and quote exactly the
received body, truncated at maxErrorBodyLen (512) with an … marker.

return fmt.Errorf("unexpected response status %s: %s", res.Status, truncateBody(body, maxErrorBodyLen))

Why

Two defects, both measured against the shipped code:

1. body[:50] reads past len(body). It does not panic, contrary to what
   one might expect: io.ReadAll always allocates with cap >= 512, so the
   re-slice stays inside the allocation and exposes the read buffer's slack
   instead. A {"error":"x"} body rendered as 404{"error":"x"} followed by 37
   NUL bytes; an empty body as 404 followed by 50 NUL bytes. The padding is
   NUL today only because the buffer is freshly allocated — the code has no
   right to those bytes either way.
2. The body was the format string. A 100% failed body rendered as
   100%!f(MISSING)ailed.

The 512-byte bound is new (the old 50 was an accident of the broken slice); it
keeps a typical JSON error payload intact while stopping a multi-megabyte HTML
error page from landing in a log line verbatim.

Tests

• TestTruncateBody — len == 0, len < limit, len == limit,
  len == limit + 1; the marker appears only in the last case.
• TestExecStatusError — 4 bodies (empty, short JSON, one containing %, one
  of 2×maxErrorBodyLen) × 2 assertion paths (ResponseCodeOk vs 404,
  ResponseCodeFail vs 500) = 8 cases. Asserted by equality on the full
  message, not Contains: the defect is extra bytes appended to the body,
  which a substring check would pass.

Red-first check: with the two branches reverted to the old expression, all 8
TestExecStatusError cases fail.

Local gates: go test -count=1 ./... (40 tests), go vet ./... — clean, which
is itself part of the proof, since vet flags the old non-constant format string
— and gofmt -l ..

Notes

• No API change; truncateBody and maxErrorBodyLen are unexported, so
  gorequests-proxy and gorequests-retry are unaffected.
• The error message text changes. Nothing in the three modules asserts on it.
• Stays within go 1.18; io/ioutil and the Go baseline are left alone
  deliberately (separate modernisation task).
• Found in passing, not fixed here: Url(format, args...) runs the URL through
  fmt.Sprintf, so a % in the URL itself (e.g. percent-escaping) is mangled
  into %!f(MISSING).

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