refactor: replace bigSegmentStatus string with EvaluationReason::BigSegmentsStatus enum

[beekld]

Replace bigSegmentStatus string with typed enum

Renames the (currently unused-but-public) EvaluationReason Big Segments field from singular bigSegmentStatus to spec-compliant plural bigSegmentsStatus, and introduces a typed enum to replace its std::optional<std::string> representation. No evaluator behavior change in this PR — the field still has no in-tree writer; that lands in SDK-2366-2.

What's in

• New nested enum EvaluationReason::BigSegmentsStatus (kHealthy/kStale/kNotConfigured/kStoreError), mirroring Kind and ErrorKind.
• friend std::ostream& operator<< for the enum.
• tag_invoke pair in the JSON serializer (mirrors Kind and ErrorKind).

Breaking changes

• EvaluationReason::BigSegmentStatus() renamed to BigSegmentsStatus() and now returns std::optional<EvaluationReason::BigSegmentsStatus> instead of std::optional<std::string>.
• The 7-arg EvaluationReason constructor's last parameter changes type to match.
• JSON field "bigSegmentStatus" renamed to "bigSegmentsStatus".

The field has never had an in-tree writer and only ever returned an empty optional / been absent from JSON, so no caller could have depended on a value.

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Note

Medium Risk
Public API and JSON field renames are breaking for C++/serialization consumers, though the field was previously unused at runtime; enum stability matters for the C API.

Overview
Big Segments status on EvaluationReason is now a typed enum instead of an optional string, with the wire/API name aligned to spec as bigSegmentsStatus.

The PR adds EvaluationReason::BigSegmentsStatus (kNone, kHealthy, kStale, kNotConfigured, kStoreError) with C-API-stable numeric values, a primary BigSegmentsStatus() accessor, and ostream / JSON tag_invoke support (field bigSegmentsStatus on serialize/deserialize). Factories and equality/<< now use the enum; absent status is kNone and is omitted from JSON.

Breaking but transitional: the old BigSegmentStatus() string getter and string-typed constructor remain as deprecated shims. Call sites and tests are updated to pass BigSegmentsStatus::kNone where appropriate. No evaluator logic changes in this PR—the status still has no in-tree writer.

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