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.sh now builds it with make -C tools/openapi build and calls ./build/openapi generate-client / ./build/openapi generate-webhook-fixtures in place of ./build/chat-manager openapi .... 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.sh end-to-end against chat master (75d154f053). The log shows make -C tools/openapi build freshly building build/openapi, and both generator calls going through ./build/openapi. The script stops at the final dotnet format because dotnet is not installed locally. Everything before that step ran (generation, fixtures, and the sed fixups).
• Ran the original generate.sh from a clean tree and compared the full working tree, excluding .git and generate.sh: 216 files, all byte-identical (diff -rq).
• make openapi does not produce the same spec on every run (see note below), so I also ran both binaries by hand on one saved copy of releases/v2/serverside-api.yaml: chat-manager openapi and openapi, each doing generate-client --language dotnet plus generate-webhook-fixtures, each into its own copy of the repo. All 217 files were byte-identical.
• I reverted all regenerated output, so this PR contains only the script change.

Note: generate-spec in chat does not produce the same serverside-api.yaml on every run. For example, one default: event type flips between export.channels.error and export.review_queue.error. This problem already exists and is unrelated to this change. The .NET output was still identical across both runs.