feat: add segment management methods to ChatClient

[mogita]

Summary

Adds the segment management methods to ChatClient: CreateSegment, UpdateSegment, AddSegmentTargets. These are the endpoints needed to build and edit the segment a campaign targets. The campaign methods were already present, but without segment create/update/add-targets the workflow could not be completed through the SDK and customers fell back to raw REST (reported by rewardstyle).

Depends on GetStream/chat#14006, which exposes the three endpoints in the OpenAPI spec (they were Ignore: true). The methods and types here match what regenerating from that spec produces; applied surgically to avoid unrelated regen drift.

Changes

• chat.go: CreateSegment, UpdateSegment, AddSegmentTargets
• requests.go: CreateSegmentRequest, UpdateSegmentRequest, AddSegmentTargetsRequest
• models.go: CreateSegmentResponse, UpdateSegmentResponse
• chat_test.go: stub smoke tests for the three methods
• chat_misc_integration_test.go: TestChatSegmentCampaignIntegration covering create segment, add targets, update, then create/start/stop a campaign (skips when the app lacks the campaigns feature)
• CHANGELOG.md: 4.2.0 entry

Testing

• go test -short -race ./... passes
• go vet ./... clean, gofumpt clean

Note

Hold merge until GetStream/chat#14006 lands and the campaigns owner confirms the payloads are intended-public.