checker: Detection of circular type references

[fleximus]

Issue: V incorrectly permitted circular type references in sum types like:
type Type6 = Type7 | int
type Type7 = Type6 | string

Root Cause: The checker only detected direct self-references (type A = A | int) but not indirect cycles through other sum types.

Fix:

1. vlib/v/checker/checker.v

Added cycle detection for sum types:

• Added an else if branch to check when a variant is itself a sum type
• Added a new helper function sumtype_has_circular_ref() that recursively traverses sum type variants to detect cycles
• Uses a visited map to prevent infinite recursion during cycle detection

2. New test files:

• vlib/v/checker/tests/sumtype_circular_reference_err.vv - Test case with circular reference
• vlib/v/checker/tests/sumtype_circular_reference_err.out - Expected error output

Verification:

The fix correctly detects:

• 2-type cycles: Type6 → Type7 → Type6
• 3-type cycles: TypeA → TypeB → TypeC → TypeA
• Deeper indirect cycles

Non-circular sum types that reference other sum types still work correctly.

Fixes #24511

[spytheman]

(rebased over current master, to force a recheck in the CI jobs running on windows)