Fix Strands activity tool error reporting

[brianstrauch]

What changed

• Report an activity failure's underlying message to the Strands model for activity-backed and MCP tools.
• Pass the underlying exception to AfterToolCallEvent hooks.
• Add workflow integration tests, documentation, and a changelog entry.

Why

Failed activities otherwise reach Strands as an ActivityError with a generic message, hiding the actual failure from the model and hooks. This matches the fix in temporalio/sdk-typescript#2467.

Validation

• poe test -s -k 'failed_activity_tool_reports_cause or activity_tool_interrupt or test_mcp or test_tool' tests/contrib/strands (8 passed)
• poe lint