fix(core): surface MCP tool error results

[WilliamK112]

Summary

• raise a typed ToolInvocationError when an MCP server returns CallToolResult.isError: true
• preserve the existing processed text result on the exception through its message and content attribute
• apply the behavior consistently across all five supported MCP protocol transports
• add regression coverage for every protocol version, with and without telemetry enabled

Validation

• 286 passed — MCP transport unit tests
• 525 passed — complete non-E2E toolbox-core suite
• black --check .
• isort --check .
• mypy -p toolbox_core
• E2E tests require the external Toolbox service and TOOLBOX_VERSION environment configured by the project validation pipeline

No test infrastructure changes are required, and no documentation changes are needed for this bug fix.

Fixes #633