checker: detect recursive .str() method calls

[medvednikov]

Summary

• Detect infinite recursive .str() method calls when .str() is called explicitly (e.g. n.str()) on the same receiver, not just through string interpolation
• Previously, only string interpolation ('${n}') inside a .str() method was caught; explicit n.str() calls caused a segfault at runtime instead of a compile error
• Add test case for sum type with recursive .str() in a match else branch

Closes #27923

[chatgpt-codex-connector]

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