fix(models): support PEP 604 unions in discriminator cache

[danielgaskins]

Fixes #20.

PEP 604 unions cannot be weakly referenced on Python 3.10 through 3.13. When response validation falls back to loose construction, the discriminator cache uses the raw union as a WeakKeyDictionary key and raises TypeError.

The cache now uses the equivalent typing.Union as its key on affected Python versions. The original PEP 604 type is still used for response parsing.

Added coverage for plain PEP 604 unions, discriminated unions, and repeated cache access.

Checked with:

• 329 network-free tests using Pydantic 2
• 316 passing tests and 13 expected skips using Pydantic 1
• Ruff
• Pyright
• Mypy