fix(UniqueGenerator): prevent memory exhaustion when chaining unique()->optional()

[teal-bauer]

Summary

• Fixes memory exhaustion when using unique()->optional()->method() or unique()->valid()->method() chains
• Adds explicit optional() and valid() methods to UniqueGenerator that properly wrap the generator chain

Problem

When calling $faker->unique()->optional()->safeEmail(), the UniqueGenerator::__call() handler would:

1. Call optional() on the underlying generator, returning a ChanceGenerator
2. Attempt to serialize() this ChanceGenerator for uniqueness tracking
3. The ChanceGenerator contains a reference to the entire Generator with all providers, causing massive serialized strings and exponential memory growth

Solution

By adding explicit optional() and valid() methods to UniqueGenerator, we ensure proper generator chaining:

• ChanceGenerator now wraps UniqueGenerator (not the base Generator)
• When the final method is called, ChanceGenerator delegates back to UniqueGenerator
• UniqueGenerator then serializes only the final value (e.g., the email string), not generator objects

Test plan

• Verified fix resolves the memory issue from unique()->optional() causes massive memory usage due to serializing Generator #1027
• Existing UniqueGenerator tests pass
• Generator-related tests pass (374 tests, 526 assertions)

Fixes #1027