doc: Fix bugs in Python introduction and interfaces overview examples

[wenzeslaus]

grass.tools snippets:

• Drop the trailing underscore from r_import_.
• Fix syntax errors in the v_info and g_region calls.

PyGRASS snippets:

• Use consistent variable names and the correct existence-check method in the Vector example.
• Import VectorTopo where it is used.
• Use the correct method names, attribute accessor, and length comparison in the line-filtering example.
• Add the missing random module import and correct the uniform argument order.
• Match the second SQL output example to its query.
• Fix typos.

All snippets were tested against the nc_basic_spm_grass7 sample dataset with Claude extracting the examples and running them.