fix: remove redundant ORDER BY in QueryForCredentials

[wyh0626]

The database-level ORDER BY is redundant since identifiers are already sorted in-memory (line 917-921 in persistence/sql/identity/persister_identity.go) Removing this improves PostgreSQL performance as it cannot use index hints.

Fixes #4520

Related issue(s)

Fixes #4520

This PR addresses a performance regression reported after upgrading from v1.3.0 to v25.4.0. Users observed:

• Significantly higher P95/P99 latencies for credential queries on PostgreSQL
• Increased database CPU usage

Root cause: The ORDER BY identifier ASC clause in QueryForCredentials is redundant because identifiers are already sorted in-memory using sort.Strings(). Additionally, PostgreSQL cannot use index hints (unlike MySQL/CockroachDB/SQLite), causing the optimizer to potentially choose suboptimal execution plans.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I am following the contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
• I have added tests that prove my fix is effective or that my feature works.
• I have added or changed the documentation.

Further Comments

This is a minimal fix that removes redundant database sorting. The in-memory sort.Strings() already guarantees deterministic ordering for testing purposes (as noted in the existing code comment).

No new tests are added because:

1. Existing tests already verify the correctness of credential retrieval
2. The change only removes redundant sorting - the final result remains identical
3. This is a performance optimization, not a behavioral change

[Pedr0Rocha]

The query plan seems ok for this specific query. I'm not sure if this will do much since the average returning rows is 1.