Fix incorrect signature with SetPreservePathSeparators(true) caused by stale m_pathHasTrailingSlash

[shurarama]

Issue #3484

This PR fixes an issue where pathless requests (e.g., STS or Athena) fail signature validation when Aws::Http::SetPreservePathSeparators(true) is enabled.

The root cause lies in the handling of URI::SetPath(...):

• When an empty path is parsed, the SDK assigns / via SetPath("/"), which sets m_pathHasTrailingSlash = true.
• Later, the SDK internally re-applies the path via uri.SetPath(uri.GetURLEncodedPathRFC3986());, assuming it is idempotent.
• But in AddPathSegments(...), this flag interferes with leading empty segment removal logic:

if (s_preservePathSeparators && m_pathSegments.empty()
    && !split.empty() && split.front().empty()
    && !m_pathHasTrailingSlash)
{
    split.erase(split.begin());
}

We reset m_pathHasTrailingSlash to false in URI::SetPath() before calling AddPathSegments(...), ensuring consistent re-parsing.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR would require a ReadMe/Wiki update.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR is a breaking (APIs have been changed) change.

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