fix: OOM crash in RemoveUnusedVariableAssignRector

[calebdw]

Closes rectorphp/rector#9733

Problem

The NoDiscard attribute check used AstResolver::resolveClassMethodOrFunctionFromCall() which, for each call expression, would:

1. Parse the target class's source file into a full AST
2. Run PHPStan scope analysis on every node (attaching heavy MutatingScope objects)
3. If the method wasn't found directly, recursively parse ALL traits via getTraits(true)
4. Retain all parsed ASTs in memory indefinitely via $parsedFileNodes cache
   For classes like Carbon\CarbonPeriod with deep trait hierarchies, this caused unbounded memory growth leading to OOM.

Fix

I replaced the expensive AST-based resolution with PHPStan's lightweight ReflectionProvider API, this checks NoDiscard via PHPStan's reflection layer without parsing any source files.

[samsonasik]

@calebdw could you rebase latest main branch so all e2e test can green, thank you.

[samsonasik]

one more change :)

#7964 (review)

[samsonasik]

Thank you @calebdw