P2: Improve MCP text response decoding and error handling

[vamgan]

Problem

decodeToolResult() parses the first text content item as JSON whenever a result looks like MCP content.

Relevant code:

• src/index.ts:136-153

It does not account for MCP isError, non-JSON text, or structured error reporting.

Impact

Consumers may receive raw SyntaxErrors or lose the distinction between tool failure and malformed responses.

Suggested fix

• Detect { isError: true } and throw a structured SDK error containing the tool result.
• Wrap JSON.parse failures with a helpful message.
• Only parse text as JSON when the response is expected to be JSON.

Acceptance criteria

• Tests cover isError: true tool results.
• Tests cover non-JSON text content with a clear SDK error.