Add MCP content ladder

The MCP adapter is token-efficient for list and search responses, but large record bodies can still become a dead end for clients that cannot reliably inspect structuredContent or expand custom transcript markers. PDPP needs a standard, grant-scoped path from compact MCP results to the full authorized record or field without dumping large bodies into every tool response.

PDPP exposes grant-scoped personal data through MCP to agent clients with uneven support for MCP response features. Current adapter work already made three important decisions:

The MCP adapter SHALL expose every truncated or non-inline record field through a bounded content ladder consisting of model-visible content[] text, canonical structuredContent metadata, a tool-callable field-window read path, and an MCP resource path when resources are available.