fix: require non-empty string values for required security secrets

[Yungmon22]

Overview

This PR tightens SecurityConfigSchema validation so required security secrets must be non-empty strings, and adds focused test coverage for the behavior described in the issue. The change is narrowly scoped to the schema's secret validation and its tests.

Related Issue

Changes

🔒 Security Config Validation

• [MODIFY] tests/utils/server-config-schema.test.ts
  • Added focused SecurityConfigSchema cases for each required secret field: sep10SigningKey, interactiveJwtSecret, and distributionAccountSecret.
  • Covers missing, empty, whitespace-only, and non-string values so invalid secret configuration fails validation.
  • Covers valid string secrets passing validation without being modified.

Verification Results

bun test

Acceptance Criteria	Status
Missing, empty, whitespace-only, and non-string secrets fail validation	✅ Covered by new SecurityConfigSchema cases per required secret field
Valid string secrets pass without being modified	✅ Covered by new SecurityConfigSchema cases per required secret field

Closes #636

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