[CHA-2961] Webhook handling spec — regenerate SDK + dual-API

[mogita]

Ticket

• https://linear.app/stream/issue/CHA-2961/webhook-handling

Related

• chat/ PR: https://github.com/GetStream/chat/pull/13350 (must merge before this can ship)

Summary

Regenerates the webhook surface from the chat/ feature/cha-2961-webhook-handling-spec branch + adds dual-API instance methods on the top-level GetStreamRuby::Client.

Added (rendered from chat/ template):

• Stream::Webhook module alias (preferred); StreamChat::Webhook retained for backward-compat for one minor-version cycle
• StreamChat::Webhook::UnknownEvent class for forward-compat
• gunzip_payload, decode_sqs_payload, decode_sns_payload primitives
• parse_event(payload) — returns typed event or UnknownEvent for unrecognized discriminators (forward-compat counterpart to the existing parse_webhook_event which throws)
• verify_and_parse_webhook(body, signature, secret) HTTP composite
• parse_sqs_payload(message_body) / parse_sns_payload(notification_body) queue composites — no signature param (backend emits no HMAC for queue messages today)
• Unified error class: StreamChat::Webhook::InvalidWebhookError covering signature mismatch, invalid JSON, missing/non-string type, gzip decompression failure, invalid base64 in queue body, malformed SNS envelope. Distinguish failure modes via the message substring or cause chain.

Hand-added on GetStreamRuby::Client:

• verify_signature(body, signature) — drops the api_secret parameter, uses @configuration.api_secret
• verify_and_parse_webhook(body, signature) — drops the api_secret parameter

Dual API: module-level methods remain available for callers needing an arbitrary secret.

Conformance fixtures under test/fixtures/webhooks/ (14 event-type buckets + _invalid/ negative cases).

Generator change: generate.sh now invokes chat-manager openapi generate-webhook-fixtures to populate test/fixtures/webhooks/.

Test plan

• make test passes (rspec suite; 12/12 green)
• Conformance fixtures generated cleanly (14 event-type dirs + _invalid/)
• Stream::Webhook and StreamChat::Webhook both resolve
• Client wrapper verify_signature returns true/false correctly with stored secret
• verify_and_parse_webhook raises InvalidWebhookError on bad signature
• Rubocop clean on lib/getstream_ruby/client.rb

Notes for review

• Pre-existing codegen mismatch: event_class_for_type references StreamChat::*Event constants but generated models live under GetStream::Generated::Models::*. This affects typed-event return for known discriminators in both the old parse_webhook_event and the new parse_event. Out of scope for this PR (template-level mapping fix needed in chat/ codegen). Signature verification, UnknownEvent fallback, queue/HTTP composites, and error handling all work correctly.
• test/webhook_test.rb is generated as Minitest and depends on minitest/autorun, which is not in this project's Gemfile. The repo's test infra is rspec under spec/. The generated Minitest file documents the surface and is not currently runnable by make test — pre-existing situation.