fix(python): forward binary tool results in HandlePendingToolCall RPC

[syf2211]

Summary

When a Python tool handler returns a ToolResult with binary_results_for_llm (e.g. base64 image data) or session_log, those fields were dropped while building the ExternalToolTextResultForLlm payload in _execute_tool_and_respond. The model only received text fields, so image tool outputs appeared unavailable.

Motivation

Fixes #1644. Users building agents with image-returning tools saw the model report it could not view images, even though direct blob attachments worked.

Changes

• Add tool_result_to_external_tool_text_result_for_llm() in python/copilot/tools.py to map ToolResult → ExternalToolTextResultForLlm, including binary result type enum conversion.
• Use the helper in CopilotSession._execute_tool_and_respond instead of an inline constructor that omitted binary_results_for_llm and session_log.
• Add unit tests covering binary forwarding and the no-binary case.

Tests

cd python && uv run pytest test_tools.py::TestToolResultToExternalToolTextResultForLlm -q
cd python && uv run ruff check copilot/tools.py copilot/session.py test_tools.py

• 2 passed
• ruff: All checks passed

Notes

• Scope is Python SDK only; Node passes the full result object through RPC already.
• Composer-2.5 code review: APPROVE