fix: [M3-10329] - IAM Parent/Child: Use User infinite query in UpdateDelegationDrawer

[abailly-akamai]

Description 📝

This PR fixes a bug where (on large accounts) the UpdateDelegateDrawer only gets the first page of users, resulting in being unable to add users from subsequent pages to the delegation.

📉 This PR also improves the drawer performance a lot since we're:

• only fetching 25 users at a time (on-demand)
• rely on API for search (no need to fetch all)

Changes 🔄

• In UpdateDelegationDrawer, make use of useAccountUsersInfiniteQuery to fetch users on demand
• Implement on-demand loading in User Autocomplete
• Switch user search to API search
• Fix tests

Scope 🚢

Upon production release, changes in this PR will be visible to:

• All customers
• Some customers (e.g. in Beta or Limited Availability)
• No customers / Not applicable

Preview 📷

Before	After
Screen.Recording.2026-02-26.at.13.59.26.mov	Screen.Recording.2026-02-26.at.13.58.15.mov

How to test 🧪

Prerequisites

Have a large parent/child account (many users - ask for being added if not available)

Reproduction steps

• Try adding a user deep in the /users record
• 🚫 User isn't found

Verification steps

• Try adding a user deep in the /users record
• ✅ User is returned by the search

Author Checklists

As an Author, to speed up the review process, I considered 🤔

👀 Doing a self review
❔ Our contribution guidelines
🤏 Splitting feature into small PRs
➕ Adding a changeset
🧪 Providing/improving test coverage
🔐 Removing all sensitive information from the code and PR description
🚩 Using a feature flag to protect the release
👣 Providing comprehensive reproduction steps
📑 Providing or updating our documentation
🕛 Scheduling a pair reviewing session
📱 Providing mobile support
♿ Providing accessibility support

• I have read and considered all applicable items listed above.

As an Author, before moving this PR from Draft to Open, I confirmed ✅

• All tests and CI checks are passing
• TypeScript compilation succeeded without errors
• Code passes all linting rules