Fix INPUT availability for diag_subspace

[Stardust0831]

Linked Issue

Ref #7719 — normalize INPUT availability into a machine-readable form and align it with parameter validation.

Unit Tests and/or Case Tests for my changes

• Commands run: cmake --build build -j2 --target abacus_pw_ser; ctest --test-dir build -V -R MODULE_IO_availability; regenerated docs/parameters.yaml and docs/advanced/input_files/input-main.md; git diff --check; repository governance check.
• Result summary: The affected target and 20 focused availability tests passed, generated documentation is consistent, and repository checks passed.
• Checks not run, with reason: None.

What's changed?

• Restrict diag_subspace availability to plane-wave calculations with ks_solver=dav_subspace.
• Add a focused registration test for the availability expression.
• Regenerate the INPUT parameter metadata and advanced input documentation.

Governance Notes

• INPUT/docs changes: The availability metadata and generated INPUT documentation are updated.
• Core module impact: Limited to INPUT availability registration and its focused test; solver implementation is unchanged.
• Exceptions requested: None.

[mohanchen]

LGTM