fix(coderd): support exact user email search - #27922

Summary

User search can now resolve exact email: and username: terms through GET /api/v2/users instead of only supporting fuzzy free-text matches. The database query already had exact email and username filters; this wires the public search parser and API handler to those filters so clients can ask for a single user by email without fetching every user or depending on substring matching.

This is the API half of coder/terraform-provider-coderd#403: that provider PR adds data.coderd_user.email, and this PR gives it an efficient exact lookup path.

Testing

• go test ./coderd/searchquery -run '^TestSearchUsers$' -count=1
• go test ./coderd -run '^TestGetUsersFilter$' -count=1
• Live API test:
  • Built local enterprise Coder from this branch.
  • Started Coder on http://127.0.0.1:39991 against a clean Postgres database.
  • Created lookup-target@example.com.
  • Verified GET /api/v2/users?q=email:LOOKUP-TARGET@EXAMPLE.COM&limit=2 returned exactly one user:

{
  "count": 1,
  "users": [
    {
      "id": "efc6f909-ce0a-4731-bd2f-6e4df417aaa7",
      "username": "lookup-target",
      "email": "lookup-target@example.com"
    }
  ]
}