feat: Pydantic v2 upgrade

[lmmx]

• Closes Support Pydantic V2 #286

I discussed the approach a little here #286 (comment)

• I fixed what I think was an incidental bug in int vs. float handling, added exceptions for ABCs and Protocols, and a fallback to an Any-typed schema (which importantly we never see in the tests, it isn't expected to fire in normal usage).
• I updated the way the validator is called, fairly straightforward.
• I moved the MissingDependency machinery inside the call site (this seemed necessary, and after this the tests worked in both with/without dateutil)
• I updated the test fixtures to the v2 (improved!) JSONSchemas, they are only v lightly changed from the old ones.
• Other trivial changes in Pydantic v2 API

Conditional imports

This PR introduces a hard import on pydantic and pydantic core in 4 modules ({_base,sized,schema,negated}.py).

• The simplest way to solve this is a conditional import that simply fails, which I did.
• Then it's best for performance to gate any imports not actually used (besides for type hints) under TYPE_CHECKING, which I did too. (For instance, someone with pydantic installed but not using the pydantic parts of this library shouldn't "pay" for the time to import pydantic)
• I also introduced a small helper utility func to raise the MissingDependency error in a nice way, without having to repeat it everywhere

Checklist

• Runs with and without Pydantic (conditional imports work)
• Tests pass with make test
• mypy check passes
• Docs build OK (bumped to 3.12)
• Goose checks pass with goose run --select=all
• Docs updated (README + Pydantic support docs site page)