[roll] Roll fuchsia [sestarnix] Add users parsing and validation to new policy parser

Integrate parsing and validation of the Users symbol table into the
new policy parser.

This:
- Defines the new User struct in new_policy/users.rs with derived Parse,
  Serialize, and Validate implementations.
- Implements Validate for UserId to verify users exist in the policy.
- Integrates the users field into NewPolicy.
- Removes the legacy User, UserData, and UserMetadata definitions from
  policy/symbols.rs.
- Updates ParsedPolicy to delegate user lookup and validation to the
  new parser.
- Simplifies user role checking in security_context.rs using the new
  type-safe RoleSet contains method.
- Adds an explicit unit test for User parsing/serialization.

Original-Original-Bug: 527401888
Test: fx test starnix-selinux-unit-tests starnix-selinux-tests sestarnix-userspace-tests

TAG: agy
CONV: a4f3c876-713c-4cce-b711-d02b2a6c035f
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1690396
Original-Original-Revision: b2be4f7f2a85b2564197135db9657038fbbab52f
GitOrigin-RevId: 430f5477621e74dc394cbb3d4a99ef9af88fbcce
Change-Id: I612cf210bc8595bdc6fe8a1717c91238114ca5c5
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1697576
Cr-Commit-Position: refs/heads/main@{#196378}