cgen: fix autofree inserting string declarations for multiple functions calls

[walkingdevel]

Fixes #17768

🤖 Generated by Copilot at cc2b0b5

Improved the autofree feature for string operations and function calls in the V compiler. Renamed a variable and a comment, updated the statement positions, and added a valgrind test.

🤖 Generated by Copilot at cc2b0b5

• Rename last_stmt_pos to stmt_before_call_expr_pos in cgen.v to avoid confusion with Gen.last_stmt_pos and clarify its role in autofree feature (link)
• Use stmt_before_call_expr_pos instead of last_stmt_pos in g.insert_at call in cgen.v to insert string declarations before call expressions that may produce strings to be freed (link)
• Update g.stmt_path_pos array in g.insert_at method in text_manipulation.v to account for added line of code and maintain correct positions for autofree feature (link)
• Change comment /*af arg*/ to /*autofree arg*/ in fn.v to make it more consistent and clear (link)
• Add test function color_code_to_rgb and call it with sample input in multiple_fn_calls.v to check for memory leaks and errors with valgrind and autofree feature (link)

[walkingdevel]

@spytheman, maybe I'll do this PR without tests? I fixed the main problem, but memory leaks that are not related to this PR do not allow me to finish.

[spytheman]

Excellent work.

[spytheman]

@spytheman, maybe I'll do this PR without tests? I fixed the main problem, but memory leaks that are not related to this PR do not allow me to finish.

Edit skip_valgrind_files in vlib/v/slow_tests/valgrind/valgrind_test.v and add vlib/v/slow_tests/valgrind/multiple_fn_calls.v there.
That will guarantee that it will compile with -autofree, to prevent regressions, but without running it under valgrind after that.

[walkingdevel]

@spytheman, maybe I'll do this PR without tests? I fixed the main problem, but memory leaks that are not related to this PR do not allow me to finish.

Edit skip_valgrind_files in vlib/v/slow_tests/valgrind/valgrind_test.v and add vlib/v/slow_tests/valgrind/multiple_fn_calls.v there. That will guarantee that it will compile with -autofree, to prevent regressions, but without running it under valgrind after that.

Thank you!