docs(spec): document MCP 1.4.0 error extraction from content[0].text in MCPToolAdapter

The MCPToolAdapter execute() section now documents the MCP 1.4.0 error
response protocol: when isError is true, the error message is extracted
from content[0].text rather than a non-standard top-level error key.
Falls back to 'unknown error' when content is absent or malformed.

This aligns the spec with the implementation fix in PR #2600 and the
approved proposal in issue #3330.

ISSUES CLOSED: #3330

docs/specification.md

@@ -22063,6 +22063,7 @@ Bridges external MCP servers into the tool model:
    - Creates a checkpoint if the tool is marked checkpointable
 
    After the MCP tool returns its `content[]` response, the adapter:
+   - **Error detection**: If `isError: true` is present in the response, the error message is extracted from `content[0].text` per the MCP 1.4.0 protocol (the non-standard top-level `error` key is not used). Falls back to `"unknown error"` when `content` is absent or malformed.
    - Parses the result into the CleverAgents `Result` format
    - Records any resource mutations as `Change` objects in the plan's `ChangeSet`