c-ares / c-ares / 28707254443

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Fix compiler warnings and add CARES_WERROR (enable -Werror in CI on clean toolchains) (#1191)

Fixes the compiler warnings emitted by the high-warning CI builds and
adds a `CARES_WERROR` build option, enabled in CI on the toolchains that
build the library cleanly — so warning regressions can no longer be
merged on those platforms.

## Warning fixes

**`ares_buf.c:1148` `[-Wsign-conversion]`** — emitted by Ubuntu (latest)
and every high-warning build.
- **Origin:** introduced by #1099 (`557139ad`), which changed the offset
calc to `(size_t)(ptr - buf->alloc_buf - buf->offset)`. The inner
subtraction mixes the signed pointer difference with the unsigned
`buf->offset`, implicitly converting signed→unsigned before the outer
cast.
- **Fix:** cast the pointer difference to `size_t` first, then subtract
the (unsigned) offset. Value is unchanged — `ptr` is always `>=
alloc_buf + offset`, so no underflow.

**`ares_event_wake_pipe.c:120` `[-Wunused-result]`** — a plain `(void)`
cast does not silence glibc's `warn_unused_result` on `write()` under
`_FORTIFY_SOURCE`. Consume the result via an (ignored) error check.

## `-Werror` in CI

New `CARES_WERROR` CMake option (default **OFF**). It applies `-Werror`
(GCC/Clang) or `/WX` (MSVC) to the **c-ares library targets only** — not
the test/fuzz harnesses (which carry their own warnings) or CMake's
feature checks.

Enabled (`-DCARES_WERROR=ON`) on toolchains verified warning-free at our
high warning levels:

| Enabled | Compiler |
|---|---|
| ubuntu-latest | gcc 13 / clang 18 |
| fedora-latest | gcc 16 / clang 22 |
| macos, ios | AppleClang 17 |
| openbsd | clang 16 |
| msys2 (MINGW64 / CLANG64) | gcc 16 / clang 22 |
| uwp | MSVC (VS 18) |

**Intentionally excluded:** ubuntu-2004 (gcc 9.4) and centos7 (gcc
4.8.5) — spurious `-Wconversion` at these levels; qnx (vendor QCC),
watcom, djgpp — old/vendor/DOS toolchains with bogus or ABI-specific
warnings.

**Follow-up (not in this PR):** systems with genuine platform-code... (continued)

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