chore: build the generator from chat's tools/openapi

[Acconut]

Ticket

• https://linear.app/stream/issue/CHA-5625

Summary

GetStream/chat#17426 moved the OpenAPI generator out of chat-manager into its own module, tools/openapi. generate-openapi.sh now builds it with make -C tools/openapi build and calls ./build/openapi generate-client in place of ./build/chat-manager openapi generate-client. Subcommands and flags are unchanged. make openapi is kept for spec generation. chat-manager openapi only remains as a compatibility path and will be removed in CHA-5626, so the script would break without this change.

Verification

• Ran the new generate-openapi.sh end-to-end (bash -x, Node v24 from .nvmrc, yarn lint:gen included) against chat master (75d154f053). Beforehand I deleted build/openapi in chat. The trace shows make -C tools/openapi build rebuilding it, then ./build/openapi generate-client --language ts --spec ./releases/v2/serverside-api.yaml.
• Snapshotted src/gen, reset it, then ran the original script. Both runs had the same releases/v2/serverside-api.yaml (checked with cmp). I compared all 10 files in src/gen and every one was byte-identical (diff -rq).
• I reverted all regenerated output, so this PR contains only the script change.