Migrate thread participants to the generated ThreadParticipant model

[gpunto]

Goal

Migrate thread participants from the hand-written DownstreamThreadParticipantDto to the generated ThreadParticipant model, together with the null-collection handling the v1 endpoints require for it to parse.

Part of AND-1291

Implementation

The generated models come from the v2 spec, whose encoder writes nil Go maps and slices as {} and [], so their collection properties are non-null. We call the v1 endpoints, whose encoder writes null instead: POST /threads sends "custom": null on every thread_participants entry.

• Add NullCollectionsAsEmptyFactory, which reads an explicit null as an empty collection for the non-null List and Map properties of the generated network models. It only claims classes in the network.models package that declare such a property, and is registered last so the model-specific adapters keep precedence and delegate into it.
• Add the generated ThreadParticipant and UserResponse, plus UserResponseAdapter for the user's flattened custom data; remove the hand-written DownstreamThreadParticipantDto.
• Retype thread_participants on DownstreamThreadDto and DownstreamThreadInfoDto and map it to the domain ThreadParticipant in DomainMapping. The message-level thread_participants is a list of users rather than participants, so DownstreamMessageDto is unchanged.

Testing

• NullCollectionsAsEmptyFactoryTest: an explicit null becomes an empty collection, an absent key still falls back to the model default, populated collections are read unchanged, and hand-written DTOs are left alone.
• ThreadParticipantParsingTest parses a payload captured from POST /threads. Removing the factory registration makes it fail with Non-null value 'custom' was null at $.custom.
• Device-probed queryThreads on the wire: 10 threads and 22 participants parsed, the parsed count matched participantCount on every thread, and per-user custom data was collected correctly.
• Checked in a minified release build with R8 full mode: the factory's Kotlin reflection survives obfuscation, so no additional consumer proguard rule is needed.
• spotlessApply, detekt, apiCheck (no public API change) and the full stream-chat-android-client unit test suite.

[github-actions]

PR checklist ✅

All required conditions are satisfied:

• Title length is OK (or ignored by label).
• At least one pr: label exists.
• Sections ### Goal, ### Implementation, and ### Testing are filled, or the PR is bot-authored.
• An issue is linked (Linear ticket or GitHub issue), or the PR is bot-authored.

🎉 Great job! This PR is ready for review.

[github-actions]

SDK Size Comparison 📏

SDK	Before	After	Difference	Status
stream-chat-android-client	5.96 MB	5.97 MB	0.01 MB	🟢
stream-chat-android-ui-components	11.26 MB	11.27 MB	0.01 MB	🟢
stream-chat-android-compose	12.74 MB	12.75 MB	0.01 MB	🟢

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud