Why
===

Any place we might encounter a `Union[...]`, we need `# type:
ignore[arg-type]` to avoid mypy failing to typecheck, due to
pydantic/pydantic#8923, acknowledging that while
pyright seems to have good inference, mypy fails to typecheck.

To better understand the issue and determine if there were any
workarounds, I created
https://github.com/blast-hardcheese/mypy-union-bug with the intent to
report upstream, before finding the ongoing discussion threads linked
here and in comments.

What changed
============

### Main

- Reflow format strings to use multiline format strings and
`indent(dedent(s, ...))`. This gives us more horizontal space instead of
trying to use C-style line continuation to avoid the 88-character limit
- Adding `# type: ignore[arg-type]` to possible `Union` slots

### 🧹 
- Tracking type literals more closely
- Reducing nested format strings for clarity
- Bubble up string-formatted comma for intelligibility

Test plan
=========

Manually running against production schemas, gathering sources together
for a sensible local test suite for codegen