build: support system fmt dependency

[wxyucs]

Summary

• Add VSAG_USE_SYSTEM_FMT as a per-dependency override for VSAG_USE_SYSTEM_DEPS.
• Reuse pre-existing or system fmt::fmt targets and fall back to bundled fmt when AUTO cannot find a valid system header.
• Add target header validation so incomplete fmt packages fail early under ON instead of breaking during compilation.

Verification

• make debug VSAG_USE_SYSTEM_DEPS=OFF EXTRA_DEFINED=\"-DVSAG_USE_SYSTEM_FMT=AUTO\"
• External superbuild at /home/xiangyu/vsag-system-fmt-superbuild with parent-provided fmt::fmt and an executable linking both vsag and fmt::fmt.

[mergify]

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[gemini-code-assist]

Code Review

This pull request introduces support for using a system-installed fmt library instead of always downloading it via FetchContent. It adds a helper function vsag_target_has_header to verify if a target contains the required header, exposes a VSAG_USE_SYSTEM_FMT option, and updates the fmt configuration to search for the system package. Feedback suggests removing the redundant and cache-polluting find_path call in favor of relying solely on find_package, and adding a safety check in vsag_target_has_header to prevent checking the root directory when an include directory path is empty.

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[Copilot]

Pull request overview

This PR extends VSAG’s CMake third-party dependency resolution to support using a system-provided fmt independently of the global VSAG_USE_SYSTEM_DEPS setting, including reuse of a parent project’s pre-existing fmt::fmt target and early validation of required headers.

Changes:

• Add VSAG_USE_SYSTEM_FMT cache option as a per-dependency override.
• Update extern/fmt/fmt.cmake to prefer pre-existing/system fmt::fmt and fall back to bundled fmt when unavailable.
• Introduce a CMake helper to validate that an imported/parent-provided target’s include dirs contain a required header.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
extern/fmt/fmt.cmake	Implements system/pre-existing fmt::fmt discovery with header validation and bundled fallback.
cmake/VSAGOptions.cmake	Adds VSAG_USE_SYSTEM_FMT option to control system fmt usage policy.
cmake/VSAGHelpers.cmake	Adds vsag_target_has_header() helper used for validating dependency targets.