feat(ai-openrouter): expose openrouter:web_fetch as webFetchTool()

[tombeckenham]

Problem

@tanstack/ai-openrouter exposes OpenRouter's web search server tool via webSearchTool() (packages/typescript/ai-openrouter/src/tools/web-search-tool.ts), but there's no equivalent wrapper for OpenRouter's openrouter:web_fetch server tool.

openrouter:web_fetch lets any tool-calling model retrieve full page content from URLs, executed server-side by OpenRouter. It supports four engines (native, openrouter, exa, parallel) — with engine: "native", requests to Claude models route through Anthropic's native web fetch. Today users have to hand-construct the tool spec and it may not route correctly through the adapter's tool converter.

Proposal

Add a webFetchTool() factory mirroring the existing webSearchTool() pattern:

import { webFetchTool } from '@tanstack/ai-openrouter/tools'

const tool = webFetchTool({
  engine: 'native' | 'openrouter' | 'exa' | 'parallel',
  maxContentTokens: 4000,
  allowedDomains: [...],
  blockedDomains: [...],
})

Implementation outline (parallels web-search-tool.ts):

• New packages/typescript/ai-openrouter/src/tools/web-fetch-tool.ts with webFetchTool(), WEB_FETCH_TOOL_KIND, isWebFetchTool(), convertWebFetchToolToAdapterFormat().
• Export from tools/index.ts.
• Route in tool-converter.ts (convertToolsToProviderFormat) so branded fetch tools are emitted in OpenRouter wire format ({"type": "web_fetch", "web_fetch": {...}} — verify against OpenRouter docs).
• E2E coverage in testing/e2e/ (per the repo's mandatory E2E policy).

References

• OpenRouter announcement: https://openrouter.ai/announcements/agentic-web-tools
• Existing pattern: packages/typescript/ai-openrouter/src/tools/web-search-tool.ts
• Anthropic-direct equivalent already exists: packages/typescript/ai-anthropic/src/tools/web-fetch-tool.ts