fix(slack): resolve actor name via direct user lookup instead of paginated list[ES-557]

[Harika Kondur (harikakondur)]

Purpose

Customer reported (ES-557) that Slack entry notifications show a raw, illegible user ID instead of the author's name — despite #11167 already adding actor name resolution.

Approach

getUserName was resolving the actor's name by fetching cmaClient.user.getManyForSpace({}) — a paginated collection — and searching for the actor in the first page of results. For spaces with more members than the default page size (e.g. large orgs synced via SSO), an actor outside that first page was never found, silently falling back to the raw ID. This matches the customer's report.

Replaced the paginated list + .find() with a direct single-user lookup: cmaClient.user.getForSpace({ spaceId, userId }). This resolves the exact actor in one call, independent of space size, and still falls back to the raw ID if the lookup fails (e.g. deleted user).

getSpaceName was checked as part of this investigation — it already uses cmaClient.space.get({ spaceId }), a direct single-space lookup, so it was not affected by this bug.

Testing steps

• Updated service.test.ts to stub user.getForSpace instead of user.getManyForSpace.
• Added a test asserting the ID fallback still works when the user lookup fails/rejects.
• npm test, tsc --noEmit, and eslint all pass on the changed files.

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