Add pydantic Field requirements to API model attributes

[Chennamma-Hotkar]

Description

Adds pydantic Field() constraints to model attributes that previously accepted any integer without validation, as described in #520.

Model	Field	Constraint	Rationale
TUFSignedDelegationsRoles	threshold	ge=1	TUF spec: ≥1 signature required
TUFSignedDelegationsSuccinctRoles	threshold	ge=1	Same
TUFSignedRoles	threshold	ge=1	Same
TUFSignedMetaFile	version	ge=1	TUF spec: versions start at 1
TUFSigned	version	ge=1	Same
ArtifactInfo	length	ge=0	Cannot be negative
BootstrapPayload	timeout	gt=0	Must be positive

Tests: 13 new unit tests, 99 total pass, tox -e lint clean.

Docs: Regenerated docs/swagger.json.

[MVrachev]

Linting is failing, have a look at the failed pipeline at the bottom of the pr.

[Chennamma-Hotkar]

Thanks for the review @MVrachev! I see the linting failure — some lines exceeded 79 characters due to the description parameter in Field(). I'll fix this by keeping the Field constraints minimal (without descriptions) and push an update shortly.

[Chennamma-Hotkar]

@MVrachev I've pushed a fix — removed Field descriptions to stay within the 79-char line limit, and all lint checks (flake8, isort, black, bandit) pass locally via tox -e lint. Please re-approve the workflow when you get a chance.

[kairoaraujo]

Hi @Chennamma-Hotkar, could you please solve the conflicts?