feat: add shared Foundry extension feature access

[Hui Miao (huimiu)]

Summary

Add shared Foundry feature access without migrating existing behavior or introducing a production feature. This is the top layer of the two-PR stack, based on #10635 (hui/foundry-feature-flags-core).

Changes

• Add extensions/featureflags.IsEnabled(ctx, client, featureID, environmentName) in the existing root Go module, backed by the shared Alpha registry and pure source evaluator.
• OR the canonical process flag, the selected environment's raw dotenv values, global Alpha config, and every service's environment map. Service hosts and project-service counts do not restrict enablement, and conflicting service maps are never merged.
• Forward explicit environment selection and return genuine loading, decoding, file-read, and transport failures. Recognize only known optional no-project/no-default-environment absence when no environment was explicitly selected.
• Add focused tests and package documentation. All three changed files are under cli/azd/extensions/featureflags/; there are no dependency, manifest, registry, session carry-over, or other runtime behavior changes.

Validation

Using Go 1.26.4:

• Focused package tests pass, including all 16 four-source combinations; 97.4% statement coverage, with the snapshot-loading/evaluation and optional-scope functions at 100%.
• Five shuffled repetitions pass, including conflicting service order and invocation isolation cases.
• go vet, package-scoped golangci-lint (zero issues), source/README spelling, copyright headers, comment lengths, and git diff --check pass.
• Race testing could not run because CGO is disabled and no C compiler is available in this Windows environment.

Compatibility

Adoption requires an azd host containing the lower PR's canonical Foundry process-opt-in launch preservation; an SDK update alone cannot restore a flag lost by an older host. No unreleased version is claimed to guarantee support. Existing host environment-loading behavior, including remote resolution or environment-name normalization, is unchanged.

[azure-pipelines]

Azure Pipelines:
26 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.