feat: include structuredContent as JSON text in every tool result

[marselsel]

Part 4 of 8 in the MCP best-practices stack. Stacked on #61, so merge in order.

Why

• The spec (2026-07-28, server/tools): "For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block."
• This server: each tool's text was a one-line summary ("Invoice X retrieved."). A client that shows the model only content never saw the document itself.
• Anthropic's tool-design reference says: "Always include the text fallback — not all hosts read structuredContent yet." A third-party report (Claude Custom Connector cannot see structured MCP tool results yyjeqhc/webcodex#456, 2026-09-14) describes a Claude custom connector passing the model only content[0].text. Claude Code is reported to forward structuredContent, so the added text does not double what the model sees there.
• The SDK serializes only non-object structuredContent itself, so object payloads need this.

Changes

• withJsonText in shared.ts: appends JSON.stringify(structuredContent) (compact) as the last text block. Error results and non-object payloads pass through untouched.
• Applied once: registerTools wraps every handler, instead of changing about 40 handlers.

Verification

• Unit tests cover:
  • the order of blocks
  • binary results, where the embedded file is kept and the metadata JSON comes after it
  • string or missing content
  • error and scalar results, which are left untouched
• A wiring test calls a real handler through registerTools.
• Over the wire, get-document-link returned the summary block followed by the JSON block.
• npm test passes all 475 tests.