chore: fix oxlint warnings (unused variables and imports)

[sicnuyudidi]

Fixes #25189

What changed

Fixed 55 oxlint warnings across 20 files in the web/ directory:

• Unused React imports (4 test files): Prefixed with _ in vi.mock callbacks
• Unused type alias (types/doc-paths.ts): Removed unused DifyDocPath type
• Useless escape chars (8 test/script files): Removed unnecessary backslashes
• Useless fallback in spread (5 files): Changed ...(x ?? {}) to ...x
• Prefer endsWith (scripts/check-i18n.js): Replaced regex with .endsWith(".json")
• Unsafe optional chaining (use-workflow-run.spec.ts): Extracted intermediate variable
• Unused expression (public/embed.js): Converted ternary to if/else block

All project source code warnings are now resolved. Remaining warnings are from third-party Monaco Editor files in public/vs/.