Fix: add missing <string> include in simple_pexsi.h (Intel classic build)

[Flying-dragon-boxing]

Reminder

• I have read AGENTS.md and docs/developers_guide/agent_governance.md.
• I have linked an issue or explained why this PR does not need one.
• I have added adequate unit tests and/or case tests, or explained why not.
• I have listed the exact verification commands run and their results.
• I have described user-visible behavior changes, including INPUT parameter changes.
• I have explained core-module impact for ESolver, HSolver, ElecState, Hamilt, Operator, Psi, or other source/ changes.
• I have requested any needed governance exception below.

Linked Issue

None — trivial header self-containment fix for an Intel classic (icpc) build failure; a dedicated issue would add no information beyond this description.

Unit Tests and/or Case Tests for my changes

• Commands run:
  • Self-containment check, post-fix: mpicxx -std=gnu++14 -fsyntax-only on a TU containing only #include "source_hsolver/module_pexsi/simple_pexsi.h" — passes.
  • Same check on the pre-fix header: fails with simple_pexsi.h:20:28: error: 'string' in namespace 'std' does not name a type — confirming the header was not self-contained and only compiled via transitive <string> includes.
  • Intel classic icpc 2021.6/2022.1 build of pexsi_solver.cpp (which previously emitted simple_pexsi.h(20): error: namespace "std" has no member "string"): verified on the Intel classic machine.
• Result summary: all pass.
• Checks not run, with reason: no unit test applicable to an #include addition; no runtime behavior change.

What's changed?

• source_hsolver/module_pexsi/simple_pexsi.h: add #include <string>. The header declares simplePEXSI(..., const std::string PexsiOptionFile, ...) but only included <mpi.h>; std::string relied on transitive includes. GCC/icpx happen to pull in <string> through other headers; Intel classic (icpc) does not, so compiling pexsi_solver.cpp fails with namespace "std" has no member "string". Including <string> explicitly follows the self-contained-header principle and is safe on every compiler.

Governance Notes

• INPUT/docs changes: none; no user-visible behavior change.
• Core module impact: HSolver/PEXSI header only; no runtime behavior change.
• Exceptions requested: none.

[Copilot]

🟢 Approval recommended

The change is a minimal, correct header self-containment fix with no behavioral impact and no additional risks identified in the diff.

Pull request overview

This pull request fixes a header self-containment issue in the HSolver/PEXSI interface by adding an explicit standard library include required by the header’s public API, addressing an Intel classic (icpc) compilation failure without changing runtime behavior.

Changes:

• Add #include <string> to simple_pexsi.h so std::string is available without relying on transitive includes.

File summaries

File	Description
source/source_hsolver/module_pexsi/simple_pexsi.h	Adds the missing <string> include to make the public declaration using std::string self-contained.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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