[Jay0130-a]

feat(plugin): add DeepSeek Search and Exchange Rates builtin plugins

What this PR does

Adds two new builtin plugins to the pluginproduct registry, following the existing structure exactly (openapi yaml + plugin_meta.yaml entry + official_plugin_icon logo):

1. DeepSeek Search (plugin_id: 23, tools 230001 / 230002)

Tools for the DeepSeek Tools API released 2026-10-17:

• deepseek_web_search (POST /v1/tools/search) — AI-native web search. Each result carries title, url, snippet, summary, site_name, date, rerank_score (0-3, ≥1.5 = quality) and authority (0-4, ≥3 = highly authoritative), so agents can filter on quality instead of trusting position order. Operators (site:, -site:, -"phrase", "exact phrase", lang:, after:, before:) are passed through as-is. Optional ori_question improves relevance; need_content optionally returns full page content.
• deepseek_web_fetch (POST /v1/tools/fetch) — fetch a single page and return title + body as clean Markdown. Retrieval/extraction happen on DeepSeek's infrastructure.

Auth: service_http Bearer — the same API key as the DeepSeek model platform (key from platform.deepseek.com).

2. Exchange Rates (plugin_id: 24, tools 240001 - 240003)

ECB daily reference rates for 30+ currencies (USD, CNY, EUR, JPY, GBP, ...) via Frankfurter:

• get_latest_rates (GET /latest) — latest rates with base / symbols filters
• get_historical_rates (GET /{date}) — rates for a specific date or a date range
• list_currencies (GET /currencies) — supported currency codes and names

Auth: none (free public API, no key required).

Why these two

• Web search/fetch and currency conversion are the most-requested agent tools; neither is covered by the current builtin set (Bocha covers search but DeepSeek Search adds scored/reranked results and page fetch; no rates plugin exists).
• The pair also exercises both auth modes (service_http and none), which is useful as reference implementations for future contributors.

Validation

• Both OpenAPI 3.0 documents pass openapi-spec-validator (including the description requirement on responses that kin-openapi's doc.Validate enforces in load_plugin.go).
• plugin_id 23/24 and product_id values are collision-free against the existing registry (next free IDs; tool_id follows the {plugin_id}{seq} convention).
• Plugin YAML structure matches the loader contract in backend/domain/plugin/conf/load_plugin.go (manifest validate → openapi3 validate → duplicate-id check).

Screenshots / Demo

Logos (generated 128x128 PNG): plugin_deepseek_search.png (indigo magnifier), plugin_exchange_rates.png (blue rising-line chart). Live-call demo can be provided on request — DeepSeek Search has already been E2E-verified against the same API in the Dify plugin submission (langgenius/dify-plugins#3398).