Migrate from Pydantic v1 to native Pydantic v2

[JosueNina]

Closes #621

Replaces all Pydantic v1 patterns with their native v2 equivalents:

• @validator -> @field_validator(..., mode="before") @classmethod
• .dict() / .json() -> .model_dump() / .model_dump_json()
• Optional[T] fields without defaults -> explicit = None
• typing.Pattern -> re.Pattern
• pydantic>=1.8.2 -> pydantic>=2.0.0 in setup.py

Adds ConfigDict(coerce_numbers_to_str=True) to WrappedBaseModel to ensure the base model continues to allow automatic number to string conversion, and patch in conftest.py that pre converts str paths to Path before pydantic-core runs, working around a pyfakefs conflict with Pydantic v2's compiled path validator.

---

All core commands were tested and verified successfully.

Command	Status
lean login / init	✅
lean create-project	✅
lean library add / remove	✅
lean backtest	✅
lean research	✅
lean optimize	✅
lean cloud push / pull	✅
lean cloud backtest	✅
lean live deploy	✅
lean cloud live deploy / stop	✅