fix(openai): return 400 for inline file limit

[NgoQuocViet2001]

💻 变更类型 | Change Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 👷 build
• ⚡️ perf
• 📝 docs
• 🔨 chore

🔀 变更说明 | Description of Change

When an OpenAI-compatible request includes more than the supported number of inline files, the preprocessing path already treats it as a request-level validation failure. However, that specific limit error was returned as a plain error, so the public API surfaced it through the generic inline-file failure path as a 500.

This change wraps the inline file limit failure as an inlineFileUploadError with 400 Bad Request, matching the existing behavior for invalid inline file payloads such as malformed data URLs.

Added a regression test that sends 51 inline image blocks and verifies:

• the response is 400 Bad Request
• the limit message is returned to the caller
• completion is not called after the validation failure

📝 补充信息 | Additional Information

Validation run in a clean LF checkout with Go 1.26.0 and golangci-lint v2.11.4:

• ./scripts/lint.sh
• ./tests/scripts/check-refactor-line-gate.sh
• ./tests/scripts/run-unit-all.sh
• npm ci --prefix webui
• npm run build --prefix webui

Focused regression check:

• go test ./internal/httpapi/openai -run InlineUpload

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