feat(schema): add --v2, so the v2 model's schema can be printed

[venugopalanvip]

The gap

schema is the only command with no way to reach the v2 models:

command	v2
lint	--v2, or detected from $schema
resolve	--v2, or detected
calldata	--v2, or detected
convert erc7730-to-eip712	detected
schema	no option — always printed v1

So erc7730 lint --v2 validates against the v2 models, but the schema it enforces could not be obtained from the tool at all.

Why that schema is the one worth printing

A descriptor in the clear-signing registry is validated twice, against two independently maintained things:

• specs/erc7730-v2.schema.json, written by hand, in the validate JSON schemas job;
• these pydantic models, by erc7730 lint, in the validate descriptors job.

When the two disagree, a descriptor passes one check and fails the other. That is exactly #338: erc7730 generate emits metadata.info.legalName, lint accepts it, and the registry's schema rejects it. Printing the model's schema is how a comparison like that gets made in the first place.

The change

A --v2 flag, matching how lint declares its own, and switching both forms — schema input --v2 and schema resolved --v2. The v2 imports are local to the branch, as in the other commands.

$ erc7730 schema input            121,912 bytes
$ erc7730 schema input --v2       123,036 bytes
$ erc7730 schema resolved          93,441 bytes
$ erc7730 schema resolved --v2     99,506 bytes

Tests

All four combinations, plus one that checks the flag actually reaches the v2 model rather than only that a schema was printed — exit code 0 would not have caught a wiring mistake here, since both models are named InputERC7730Descriptor:

assert "excluded" in v1 and "mustMatch" not in v1
assert "mustMatch" in v2 and "excluded" not in v2

mustMatch is a v2 visibility rule and excluded a v1 field, so each name appears in exactly one of the two schemas.

One thing worth knowing

Both models carry the same title (InputERC7730Descriptor), so the two schemas are not self-identifying — only their contents differ. Anyone diffing them has to track which is which. Renaming a model's title is a wider change than this flag, so I have left it; worth a follow-up if you would rather the output named its version.