CBG-5311: [4.0.5 backport] Cleanup for GetUser

[gregns1]

CBG-5311

Backports #8259

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/0000/

[Copilot]

Pull request overview

Backport of upstream “Cleanup for GetUser” changes to make auth/session and REST auth flows safer when GetUser/GetRole can return (nil, nil), and to normalize concurrent registration behavior.

Changes:

• Hardened multiple call sites against GetUser/GetRole returning nil (avoid nil dereferences; return appropriate auth errors).
• Updated RegisterNewUser to treat CAS-race “user already created” as success (return existing user with nil error) and simplified callers accordingly.
• Improved error handling/logging paths where user data was previously referenced via potentially-nil principals.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
rest/session_api.go	Simplifies registration error handling to align with updated RegisterNewUser CAS-race semantics.
rest/handler.go	Ensures GetUser returning nil after principal updates results in ErrInvalidLogin instead of leaving h.user nil.
rest/diagnostic_api.go	Avoids dereferencing user.Name() when GetUser returns an error by using the requested username string.
db/database.go	Skips missing roles/users during sequence regeneration to avoid nil principal usage.
db/crud.go	Handles GetUser returning nil when reloading the active user, with a warning.
auth/session.go	Treats missing user as invalid session (prevents nil deref on user.GetSessionUUID()).
auth/auth.go	casUpdatePrincipal returns not-found when a principal disappears during CAS retries; RegisterNewUser returns existing user with nil error on CAS-race.