cgen: fix auto str for interface struct member which implements str method

[felipensp]

Fix #18547

🤖[deprecated] Generated by Copilot at e2df780

This pull request implements and tests the feature request #10464, which enables automatic str methods for interfaces. It modifies the vlib/v/gen/c/str.v and vlib/v/gen/c/auto_str_methods.v files to generate correct C code for calling str methods on interface values, and adds a new test file vlib/v/tests/interface_auto_str_test.v to verify the expected behavior.

🤖[deprecated] Generated by Copilot at e2df780

• Implement automatic str methods for interfaces (link, link, link, link)

• Add a special case for interfaces with str methods in vlib/v/gen/c/auto_str_methods.v (link)
• Handle DumpExpr for interfaces and other types with custom str methods in vlib/v/gen/c/str.v (link, link)
• Add a test file vlib/v/tests/interface_auto_str_test.v to check the expected output of the str and dump methods for interfaces and structs (link)

[spytheman]

Excellent work.