Docs MCP tool failures omit isError #1600

Docs MCP catches tool execution failures and returns JSON error text without isError: true. MCP clients therefore receive a successful tool result containing an error message.

For example, calling get-available-features without OPENROUTER_API_KEY returns an error envelope without the standard error flag. Exceptions from search-docs follow the same path. The SDK returns these caught results unchanged.

The shared errorEnvelope helper should include isError: true. This behavior is inherited from the original Worker.

Acceptance criteria:

• Results returned through errorEnvelope include isError: true and retain their error text.
• Protocol-level tests cover a missing API key and a search-docs execution failure without calling a model.
• Successful tool results and the separate /internal/docs HTTP contract remain unchanged.

Separate follow-up to the Docs MCP port (#1597). Keep this behavioral fix outside the migration PR.