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 external integrators, though in-tree usage was never populating the field; evaluation behavior is unchanged.

Overview
Replaces the Big Segments field on EvaluationReason with a typed, C-API-stable enum and aligns naming with the spec (bigSegmentsStatus).

EvaluationReason::BigSegmentsStatus is added (kNone, kHealthy, kStale, kNotConfigured, kStoreError) with ostream support. The main constructor and factories now take/store that enum; BigSegmentsStatus() returns the enum (defaulting to kNone when no Big Segment query occurred). The old BigSegmentStatus() string accessor and string-typed constructor remain as deprecated shims.

JSON (de)serialization switches from "bigSegmentStatus" (free string) to "bigSegmentsStatus" with validated enum strings; kNone is omitted on the wire. operator== and debug printing follow the new field.

Call sites and tests are updated accordingly. No evaluator logic change in this PR—the status still is not written by evaluation yet.

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