feat: implement disabled flag evaluation (reason=DISABLED)

[jonathannorris]

Context

Today, evaluating a disabled flag in flagd returns an error (reason=ERROR, errorCode=FLAG_DISABLED). This conflicts with the OpenFeature spec, which defines DISABLED as a successful resolution reason, not an error. FLAG_DISABLED is also not a valid error code in either the OpenFeature spec or the OFREP evaluationFailure schema.

ADR: docs/architecture-decisions/disabled-flag-evaluation.md

Work

• flagd core evaluator + gRPC v1/v2 + OFREP service layers — feat: disabled flag evaluation returns reason=DISABLED #1966
• flagd-testbed: add disabled flag evaluation test scenarios — feat: add disabled flag evaluation test scenarios flagd-testbed#375
• js-sdk-contrib: flagd-core in-process evaluator — feat: flagd-core in-process evaluator returns reason=DISABLED for disabled flags js-sdk-contrib#1552
• java-sdk-contrib: flagd-core in-process evaluator — feat: flagd-core in-process evaluator returns reason=DISABLED for disabled flags java-sdk-contrib#1799
• python-sdk-contrib: openfeature-flagd-core in-process evaluator — feat: flagd-core in-process evaluator returns reason=DISABLED for disabled flags python-sdk-contrib#393
• go-sdk-contrib: flagd provider in-process mode — feat: flagd provider in-process mode returns reason=DISABLED for disabled flags go-sdk-contrib#887
• dotnet-sdk-contrib: flagd provider in-process mode — feat: flagd provider in-process mode returns reason=DISABLED for disabled flags dotnet-sdk-contrib#653

Notes

This is a breaking behavioral change — clients that branch on FLAG_DISABLED errors or HTTP 404 responses from OFREP single-flag evaluation will need to update. Ships as a minor version bump (flagd is pre-1.0). See the ADR for migration guidance.

The core evaluator and service layer changes must ship before the in-process provider updates so integrators see consistent behavior across all evaluation surfaces.