perf(query): speed up ListUsers login name equality filters (#12460)

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## Summary

`ListUsers` with `LoginNameQuery` + equals/equals-ignore-case was very
slow on large orgs (e.g. ~330k users) because the generated SQL filtered
the `projections.login_names3` **view** on the computed
`login_name_lower` expression and correlated that subquery on
`users14.id`. Postgres nested-looped every user and never used
`login_names3_users_search (instance_id, user_name_lower)`.

This change rewrites the **query planner** for that hot path: when an
equals/equals-ignore-case login-name filter is present (and not under
`OR`/`NOT`), the user list query **INNER JOINs** an indexed matches
subquery instead of filtering via the view expression. The matches SQL
mirrors `user_by_login_name.sql` (`user_name_lower` / domain paths +
`preferred` / `is_primary`).

Non-equals methods and OR combinations keep the previous view-based
filter so semantics stay unchanged.

Also adds a k6 use case that mirrors login v2 discovery
(`loginNameQuery` EQUALS_IGNORE_CASE + `organizationIdQuery`, `limit:
2`):

```bash
cd benchmark
make users_by_login_name USER_AMOUNT=100000 VUS=10 DURATION=60s
```

## Approach

1. `NewLoginNameSearchQuery` for equals / equals-ignore-case returns a
marker `loginNameEqualsFilter` (other methods unchanged).
2. `prepareUsersQuery` extracts that marker when safe, then:
- builds the usual `sq.SelectBuilder` **without** the login-name view
predicate
- adds `JoinClause` to `user_login_name_matches(.sql)` /
`_case_sensitive.sql` as `login_name_matches`
- keeps metadata JOIN/`DISTINCT` only when metadata filters are present
(same as before)
3. Embedded SQL files under `internal/query/` for the matches subquery.

Local smoke against ~330k synthetic users: baseline ~1392ms → rewritten
path ~0.4ms for a single equals-ignore-case lookup.

## Test plan

- [x] `go test ./internal/query/ -run
'TestLoginName|TestUsers|TestUserByLoginName'` (after generate-stubs)
- [x] Existing `user_test` expected SQL updated (no always-on metadata
join; login-name equals uses JOIN)
- [ ] Run k6 before/after on a large `USER_AMOUNT` (e.g. 50k–100k+) and
compare `list_users_duration` p50/p95/p99
- [x] Manual login v2 username discovery against a large org
- [x] Confirm OR / NOT / CONTAINS login-name queries still return
expected results

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---------

Co-authored-by: Cursor Agent <[email protected]>
Co-authored-by: Silvan <[email protected]>

internal/query/user_test.go

@@ -244,7 +244,7 @@ var (
 		"count",
 	}
 	usersQuery = `SELECT *, COUNT(*) OVER () FROM (` +
-		`SELECT DISTINCT projections.users14.id,` +
+		`SELECT projections.users14.id,` +
 		` projections.users14.creation_date,` +
 		` projections.users14.change_date,` +
 		` projections.users14.resource_owner,` +
@@ -278,7 +278,6 @@ var (
 		` FROM projections.users14` +
 		` LEFT JOIN projections.users14_humans ON projections.users14.id = projections.users14_humans.user_id AND projections.users14.instance_id = projections.users14_humans.instance_id` +
 		` LEFT JOIN projections.users14_machines ON projections.users14.id = projections.users14_machines.user_id AND projections.users14.instance_id = projections.users14_machines.instance_id` +
-		` LEFT JOIN projections.user_metadata5 ON projections.users14.id = projections.user_metadata5.user_id AND projections.users14.instance_id = projections.user_metadata5.instance_id` +
 		` LEFT JOIN LATERAL (SELECT ARRAY_AGG(ln.login_name ORDER BY ln.login_name) AS login_names, MAX(CASE WHEN ln.is_primary THEN ln.login_name ELSE NULL END) AS preferred_login_name FROM projections.login_names3 AS ln WHERE ln.user_id = projections.users14.id AND ln.instance_id = projections.users14.instance_id) AS login_names ON TRUE` +
 		` WHERE projections.users14.instance_id = $1 ORDER BY projections.users14.id DESC` +
 		`) AS results`