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fix: return MCP-spec tool results (isError, structuredContent) (#719)

* fix: return MCP-spec tool results (isError, structuredContent)

Tool results wrapped every payload in a custom `{ ok, result }` / `{ ok, error }`
envelope and never set `isError`, so every failure -- a locked wallet, a node
error, a business-logic error -- reached clients as a successful tool call.

Per the MCP spec, tool execution errors are reported with `isError: true`, and
structured payloads belong in `structuredContent` (a JSON object) mirrored by a
serialized text block. There is no `ok` field in the protocol.

- fail(): sets `isError: true`, message in a text block (still redacted).
- ok(): text block always; `structuredContent` when the result is a JSON object.
  Arrays and scalars ride the text block alone rather than in an invented
  wrapper, since the spec requires structuredContent to be an object.
- Results are typed as the SDK's `CallToolResult` instead of an untyped literal.
- structuredContent is derived from the serialized text, so keys with undefined
  values (e.g. an absent mimeType) are dropped by both, not one.

No outputSchema: the spec says a server MUST conform to a declared output
schema, but handlers return `unknown` from a Keymaster cast to `any`. Per-tool
output schemas and typed handler returns are a follow-up.

Closes #691

BREAKING CHANGE: MCP tool results no longer use the `{ ok, result }` /
`{ ok, error }` envelope. Failures set `isError: true` with the message as text;
successes return the payload as JSON text plus `structuredContent` for objects.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix: gate structuredContent on the serialized payload

A JS object can serialize to a JSON scalar -- a Date, or anything with a
toJSON() returning a non-object -- so testing the raw result let a string
through as structuredContent, which MCP requires to be a JSON object. Gate on
the parsed payload instead, so the object guarantee ... (continued)

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