Add integration coverage for JSON-encoded string heading targets

Exercises the #315 accommodation end-to-end through the real MCP
transport and SDK-side Zod validation: a heading target sent as the raw
JSON text of an array (as anyOf-challenged clients do) resolves the same
section for vault_read and applies the same patch for vault_patch as the
native array form, and the bare-string rejection now names anyOf client
support as the likely cause.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>