refactor(cognito): extract user-status wire values into constants

[vieiralucas]

Summary

Cognito user account states (UNCONFIRMED, CONFIRMED, FORCE_CHANGE_PASSWORD, RESET_REQUIRED, etc.) were scattered as bare string literals across service/auth.rs, service/users.rs and service/mod.rs. The auth state machine has ~20 transition points that set one of these.

Introduces crate::user_status with a named constant for each wire value. Production call sites now use the constant, which means:

• Typos in status names fail at compile time, not at runtime with a silent wire-format drift.
• Grepping for every place that sets a particular status is one command (grep 'user_status::FORCE_CHANGE_PASSWORD') instead of needing to know the exact string.

Test-only assertions that verify the wire format are left as literals on purpose — those tests want to assert against the exact wire value, not the constant's definition.

Scope note: this PR only const-ifies Cognito user statuses. Other audit-flagged candidates (KMS policy version 2012-10-17, ElastiCache statuses available/primary, Kinesis stream statuses) were considered but left as literals — they're well-known AWS wire values where a named constant would add import noise without improving readability or catching real bugs.

Test plan

• cargo build --workspace
• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p fakecloud-cognito — 72 passed, 0 failed
• CI green

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Summary by cubic

Extracted Cognito user-status wire values into crate::user_status constants to replace scattered string literals and prevent typos. No behavior change; wire JSON stays the same.

• Refactors
  • Added crate::user_status with constants (e.g., CONFIRMED, UNCONFIRMED, FORCE_CHANGE_PASSWORD, RESET_REQUIRED).
  • Replaced literals in service/auth.rs and service/users.rs with these constants.
  • Kept test assertions as string literals to validate exact wire values.

^{Written for commit 6b1e747. Summary will update on new commits.}

[cubic-dev-ai]

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