fix(mcp): treat CallToolResult isError as tool failure - #16988

Issue for this PR

Closes #16969

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When an MCP tool returns CallToolResult with isError: true, this PR throws an error from the MCP tool execution path instead of returning it as a successful tool result.

Before this change:

• MCP isError=true responses were returned through normal result flow.
• Session processor handled them as tool-result and set state.status = "completed".

After this change:

• MCP isError=true responses are converted to thrown errors with extracted text.
• Session processor follows the tool-error path.
• Tool part status maps to error as expected.

How did you verify your code works?

• Confirmed MCP dynamic tool execute path now checks result.isError.
• Added conversion of MCP content payload to a readable error message.
• Verified successful MCP calls still return normal results unchanged.

Screenshots / recordings

Not included (logic change only).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR