Reject negative DeePKS output frequency

[Stardust0831]

Reminder

• I have read AGENTS.md and docs/developers_guide/agent_governance.md.
• I have linked the reset audit issue and the original feature PR.
• I have added focused real-INPUT regression coverage.
• I have listed the verification results below.
• I have described the deepks_out_freq_elec behavior change.
• Core-module impact is limited to source_io input validation and tests.
• No governance exception is requested.

Linked Issue

Related to #7719. Follow-up to the deepks_out_freq_elec feature introduced in #6325.

Unit Tests and/or Case Tests for my changes

• Before the change, the regression test showed that deepks_out_freq_elec -1 was accepted and silently rewritten to 0.
• MODULE_IO_read_input_serial and MODULE_IO_read_item_serial pass.
• Real-INPUT coverage verifies omitted/default zero, explicit zero, negative rejection, the existing missing-deepks_out_base diagnostic, and preservation of a positive frequency through to the existing DeePKS build guard.
• git diff --check passes.
• No DeePKS runtime case was run. This PR does not change enabled-path output cadence; the focused input test verifies the existing feature guard without pretending DeePKS is enabled.

What changed?

When an INPUT file specified a negative deepks_out_freq_elec, the reset callback silently changed it to 0. Because zero disables per-electronic-step DeePKS output, an invalid or mistyped negative value looked like an intentional request to disable the feature.

This PR removes that silent reset and rejects negative values with a direct diagnostic. Omitted and explicit zero remain valid, and positive values retain the existing deepks_out_base and deepks_out_labels requirements. Other DeePKS resets and output cadence logic are unchanged.

Governance Notes

• INPUT/docs changes: no generated documentation change is needed. Parameter type, default, description, and availability are unchanged; the existing metadata already describes operation only for positive frequencies.
• Core module impact: limited to source_io input validation and focused tests. DeePKS runtime calculations and output cadence are unchanged.
• Exceptions requested: none.

[AsTonyshment]

LGTM. Negative deepks_out_freq_elec values are now rejected instead of being silently rewritten to zero, while the existing disabled and positive-frequency behavior remains unchanged.