Fix _snwprintf_s size argument in BugReportTool EventViewer (#48398)

## Summary

Caught while reading through `BugReportTool` for an unrelated review:
the two `_snwprintf_s` calls in `EventViewer.cpp` pass `sizeof(buff)` as
the buffer-size argument, but `buff` is a `wchar_t[1000]`.
`_snwprintf_s` measures its size and count arguments in **wide
characters**, not bytes, so the current code advertises a 2000-wchar
destination for a buffer that only holds 1000.

`cpp
wchar_t buff[1000]; // 2000 bytes, 1000 wchars
memset(buff, 0, sizeof(buff));
_snwprintf_s(buff, sizeof(buff), fmt, ...); // <-- 2000 passed as wchar
count
`

If the formatted output ever exceeds 1000 wchars, the Secure CRT bounds
check fires (in debug) and - depending on which `_snwprintf_s` overload
the compiler selects against the safe template - it can write past the
end of the stack buffer in release. Neither format string here is likely
to produce 1000+ characters in practice (one substitutes a process name,
the other a channel name + integer), so this is more of a latent footgun
than a known crash, but the bounds are simply wrong.

## Fix

Use `_countof(buff)` for the size argument (which is what `_snwprintf_s`
actually wants - element count, not byte count) and pass `_TRUNCATE` for
the count so output is safely capped at 999 wchars plus the null
terminator:

`cpp
_snwprintf_s(buff, _countof(buff), _TRUNCATE, fmt, ...);
`

Applied to both `GetQuery` and `GetQueryByChannel`.

## Scope

Searched the rest of the repo for the same pattern (`_snwprintf_s(buf,
sizeof(...))` / `_snprintf_s(buf, sizeof(...))`) - these two call sites
are the only occurrences in the codebase.

## Validation

- `BugReportTool.sln` rebuilds clean locally (Release|x64) and produces
`PowerToys.BugReportTool.exe`.
- No behavior change on the happy path - both formats are well under
1000 wchars in normal use.

## Risk

Low. Two-line change in a single utility that builds event-log queries
for bug reports. Truncation on overflow is strictly safer than the prior
behavior.

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

tools/BugReportTool/BugReportTool/EventViewer.cpp

@@ -45,7 +45,7 @@ namespace
         {
             wchar_t buff[1000];
             memset(buff, 0, sizeof(buff));
-            _snwprintf_s(buff, sizeof(buff), QUERY_BY_PROCESS.c_str(), PERIOD, processName.c_str());
+            _snwprintf_s(buff, _countof(buff), _TRUNCATE, QUERY_BY_PROCESS.c_str(), PERIOD, processName.c_str());
             return buff;
         }
 
@@ -53,7 +53,7 @@ namespace
         {
             wchar_t buff[1000];
             memset(buff, 0, sizeof(buff));
-            _snwprintf_s(buff, sizeof(buff), QUERY_BY_CHANNEL.c_str(), channelName.c_str(), PERIOD);
+            _snwprintf_s(buff, _countof(buff), _TRUNCATE, QUERY_BY_CHANNEL.c_str(), channelName.c_str(), PERIOD);
             return buff;
         }