fix: Ensure that serialization of a variation or rollout uses the correct overload.

[kinyoklion]

The was boost::json handles tag_invokes seems to have changed slightly such that serialization of a VariationOrRollout may be handled by the built-in handling for a std::variant instead of our specialized override.

This PR introduces the use of context to ensure our specific serialization is always used in this case.
https://www.boost.org/doc/libs/1_87_0/libs/json/doc/html/json/conversion/contextual_conversions.html

---

Note

Use Boost.JSON contextual conversions (with version guards) to ensure Flag::VariationOrRollout always serializes via the custom tag_invoke instead of std::variant defaults.

• Serialization:
  • Add VariationOrRolloutContext and a context-aware tag_invoke overload for data_model::Flag::VariationOrRollout, plus a forwarding overload without context.
  • Use the context-aware serializer when writing flag.fallthrough.
  • Extend WriteMinimal to accept a conversion context and call boost::json::value_from(val, ctx) for Boost >= 1.83; guarded by BOOST_VERSION.
  • Include boost/version.hpp to enable version checks.
• Tests:
  • Update VariationOrRollout serialization tests to call value_from(..., VariationOrRolloutContext()) for Boost >= 1.83 and adjust variant construction.

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[kinyoklion]

This is effectively a "tag" which can be used as a discriminator.

[kinyoklion]

When the situation arose this function was not getting called for serialization. Instead the standard handling for std::variant was being called. This resulted in the value being directly serialized. So for example 42 instead of {variant: 42}. This would only happen for fallthrough. A rule directly serializes the value into a root field.

[kinyoklion]

Tests will fail until the PR with build changes is merged.