Python: Propagated MCP isError flag through function middleware pipeline

[dmytrostruk]

Motivation and Context

Resolves: #1811

• Raised ToolExecutionException in MCPTool.call_tool() when the MCP server returns isError: True, which was previously silently discarded
• Added a ToolExecutionException re-raise clause to prevent the error from being swallowed by the generic exception handler
• Fixed an existing test that used MagicMock instead of CallToolResult, causing a falsely truthy isError

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	420	63	85%	97–98, 108–113, 124, 129, 175–176, 186–191, 201–202, 254, 263, 326, 334, 485, 552, 587, 589, 593–594, 596–597, 651, 666, 684, 725, 830, 843–848, 870, 923–924, 930–932, 951, 976–977, 981–985, 1002–1006, 1150
_skills.py	289	8	97%	578, 617–618, 691, 696, 848–849, 1057
TOTAL	22676	2819	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
4695	20 💤	0 ❌	0 🔥	1m 22s ⏱️

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