fix(knowledge): distinguish duplicate files by type

[koishi514-Z]

Description

Fixes duplicate-file detection for file uploads with identical content but different file types.

File deduplication now matches on type, file_hash, and file_type, allowing byte-identical files such as .md and .txt to coexist in the same knowledge base while continuing to reject duplicate uploads of the same file type.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Fixes #2374

Testing

go test ./internal/application/repository -run '^TestCheckKnowledgeExists_FileHashIsScopedByFileType$' -count=1 -v

Verified:

• Same hash with different file types (md and txt) is not treated as a duplicate.
• Same hash with the same file type remains a duplicate.
• git diff --check passes.

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable (for Go: golangci-lint run --new-from-rev=origin/main ./...)
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

no user-visible UI changes.