Add tests for synthetic ID template error paths

[prk-Jr]

Description

Synthetic ID generation in crates/common/src/synthetic.rs exercises error paths for invalid templates and signing operations, but those paths are not directly tested.

Done when

• Add a test for invalid template syntax returning a synthetic ID error.
• Add targeted coverage for error-path propagation in the template/signing flow.
• Keep the existing happy-path behavior unchanged.

Affected area

• Core (synthetic IDs, cookies, GDPR)

Additional context

Source: test_gap.md, section Synthetic ID Generation Errors.
Parent issue: #396