fix: restore sibling errors on union branch success (Python, PHP)

[clemensv]

Summary

Fixes #160 — InstanceValidator union-type branch wiped accumulated sibling errors when a union member validated successfully.

Root Cause

In both the Python and PHP implementations, the union validation loop set self.errors = [] before trying each branch type. On a successful branch match, the code broke out of the loop without restoring the backup, leaving the error list empty and discarding all previously collected violations for sibling properties.

Fix

Always capture branch errors in a local variable and restore the backup before checking success — both on match and non-match paths. This ensures sibling errors are preserved regardless of union validation outcome.

Other SDKs

Checked all 11 SDK implementations:

• Rust, Go, TypeScript, Swift — already correct (use separate temp result/context)
• C, Perl — use simple type-match functions without shared error accumulation
• Java, C# — don't handle type unions at the instance validation level

Tests

Added regression tests in both Python and PHP covering the exact reproducer from the issue (object with failing int64 siblings + succeeding ["double", "null"] union property).