feat(youcom): add You.com integration - #8584

Summary

• Add You.com integration with 8 operations covering every endpoint in their current API reference: Search, Get Contents, Answer, Research, Get Research Task, Finance Research, Search Images, Get Account Balance
• Search supports snippets, query-relevant highlights, or full page content (markdown/html, blend/cache/fetch), plus knowledge results, freshness, country, language, safesearch, and include/exclude/boost domains
• Research supports all effort levels, structured output via JSON Schema, source control, and background mode; Frontier always runs in the background and Get Research Task fetches the result
• Answer, Research, and Finance Research project their model-consumed inputs through request.modelInput
• Answer gets its own language list (it rejects ZH/JA/PT, which Search accepts), so a hidden Search language never leaks into Answer requests
• Synchronous Exhaustive research gets a 6 min request deadline since You.com documents it at up to 300s, the default outbound cap
• Skipped on purpose: deprecated livecrawl/livecrawl_formats (superseded by extraction) and the research SSE stream endpoint
• Add the You.com icon (hexagon mark only, not the wordmark), block meta with templates and skills, and the docs page intro

Type of Change

• New feature

Testing

• Validated every tool's URL, method, auth header, request fields, and response paths against You.com's published API reference (re-fetched and diffed before shipping, unchanged)
• bun run test (34,645 passed), bun run lint, bun run type-check, bun run check:audits (58/58), docs-manifest:check, check-block-registry.ts origin/staging, check-canvas-sentences.ts --block=youcom (8/8)
• Not exercised against the live API (no key available)

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)