fix: correct MCPServer.call_tool return type and remove dead code

[Ar-maan05]

Description

This PR addresses the dead-code path and incorrect return type annotation on MCPServer.call_tool() described in #2695.

1. Removed Dead Code: In src/mcp/server/mcpserver/server.py, the isinstance(result, dict) check inside _handle_call_tool() is unreachable. As traced in the issue, self.call_tool() invokes the tool manager with convert_result=True, which delegates to FuncMetadata.convert_result. This converter function never returns a raw dict (returning CallToolResult, Sequence[ContentBlock], or tuple[Sequence[ContentBlock], dict[str, Any]] instead).
2. Removed Unused Import: Removed the now-unused import json from src/mcp/server/mcpserver/server.py.
3. Corrected Type Annotation: Updated MCPServer.call_tool()'s return type signature to accurately reflect the actual runtime types:
   CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]
4. Migration Guide: Added breaking change documentation to docs/migration.md to assist developers migrating to v2.

Related Issues

Closes #2695

Verification

• Verified that the test suite passes successfully (1,194 passed).
• Verified type checking passes cleanly with pyright (0 errors, 0 warnings).
• Ran coverage and strict-no-cover to verify that code coverage remains at 100.00% and no unused coverage pragmas exist.