docs(router): add File Inputs and Web Fetch feature pages - #433

What changed

• New page router/features/file-inputs.md. Covers attaching files inline on POST /v1/chat/completions ({"type":"file"}) and POST /v1/messages ({"type":"document"}, base64 source):
  • when the provider reads files itself, the file is forwarded as sent;
  • otherwise the Router extracts the text of PDFs (text layer) and UTF-8 text files and reports "file_parse" in x_0g_trace.request_rewritten;
  • limits: 20 files, 200,000 bytes of text per file, 20 MB body;
  • the five 400 codes;
  • no file fee.
• New page router/features/web-fetch.md. Covers the {"type":"0g:web_fetch"} tool:
  • options: max_uses, max_content_tokens, allowed_domains, blocked_domains, and the openrouter:web_fetch form;
  • the round loop with its 5-round cap, fetch failures going back to the model, your own tools alongside it, and streaming behaviour;
  • billing: $0.001 per page, web_fetch_cost, and the per-round -wfN usage entries.
• features/chat-completions.md: a pointer to each page.
• sidebars.ts: added both pages under Router › Features.

The style follows the open Web Search page (#421). The two pages do not link to it, so the build does not depend on #421 being merged.

Verification

• Every example ran as written against staging, with only the host and the key changed:
  • chat + txt file: 200, correct answer, request_rewritten: ["file_parse"]
  • messages + txt document: 200, correct answer, request_rewritten: ["file_parse"]
  • chat + web fetch: 200, correct answer, web_fetch_cost: "2000" (2 pages)
  • messages + web fetch: 200, correct answer, web_fetch_cost: "2000" (2 pages)
• Error codes and envelopes were spot-checked on staging: unsupported_file_type on chat, and unsupported_file_source on messages.
• Every statement was reviewed against the 0g-router source on main in two rounds. The last round found nothing at HIGH or MEDIUM.
• pnpm build and cspell pass.

Before merging

Both features are on in staging only. On mainnet, features.file_parts, features.web_fetch_tool and search.fetch_price_usd are not set yet; the page price is $0.001 on staging. Merge this PR after mainnet turns the flags on, and check that the mainnet page price matches the $0.001 stated here.

Deliberately left out

• /v1/responses (input_file): the Responses API is not documented publicly.
• Sealed (E2EE) requests.
• The fetch backend, and rare edge cases.

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