CoPilot comment: Fix dangling `string_view` in `TreeRegexp` copy constructor/assignment

[daantimmer]

GroupBuilder stores a std::string_view pattern that references the originating TreeRegexp::storedPattern. The current copy constructor and copy-assignment operator shallow-copy rootGroupBuilder, which means the new object's group builder tree still points into other.storedPattern rather than into this->storedPattern.

If other is destroyed (or its storedPattern is moved/mutated via copy-assignment) before the copy is used, the string_views dangle.

Fix: Rebuild the group tree from the freshly stored pattern in both the copy constructor and copy-assignment operator:

TreeRegexp::TreeRegexp(const TreeRegexp& other)
    : storedPattern{ other.storedPattern }
    , regexStrategy{ CreateRegexStrategy(storedPattern) }
    , rootGroupBuilder{ CreateGroupBuilder(storedPattern) }  // rebuild from this->storedPattern
{}

TreeRegexp& TreeRegexp::operator=(const TreeRegexp& other)
{
    storedPattern = other.storedPattern;
    regexStrategy = CreateRegexStrategy(storedPattern);
    rootGroupBuilder = CreateGroupBuilder(storedPattern);  // rebuild from this->storedPattern
    return *this;
}

This mirrors what the primary constructor does and ensures all string_views point into this->storedPattern.

File: cucumber_cpp/library/cucumber_expression/TreeRegexp.cpp (lines ~191–207)