[Arguments] Fix ReplaceArgumentDefaultValueRector re-applying already-migrated imported class constant

[TomasVotruba]

Fixes #9898

Problem

ReplaceArgumentDefaultValueRector had two related defects when a scalar valueBefore is migrated to a class constant valueAfter (e.g. 'lax' -> Cookie::SAMESITE_LAX):

1. Re-applies on already-migrated code. After the first run the argument holds Cookie::SAMESITE_LAX, which resolves back to 'lax', so the rule matched it again and rewrote the constant to a fully qualified re-fetch on every run. The existing idempotency guard only covered self::/static::/parent:: constants (isSpecialClassName()), not imported/FQ class constants.

2. Change not reported in applied rules. processReplaces() mutates the node in place and returns the same instance, but the caller checked $replacedNode !== $currentNode - always false - so valid migrations were applied to the AST without being attributed to the rule (diff shown, empty "Applied rules"). The sibling FunctionArgumentDefaultValueReplacerRector already uses the correct instanceof Node check.

Fix

• Drop the isSpecialClassName() requirement from the already-at-target guard so imported class constants are skipped too.
• Report a change whenever processReplaces() returns a non-null node, matching the sibling rule.

Test

Added skip_imported_class_const_already_replaced fixture asserting an argument already holding the target imported constant is left untouched.