Fix Length constraints for object-valued Marshmallow fields

[xiehuanyi]

Length validators on Dict fields currently generate minLength/maxLength, and object-valued Nested fields generate minItems/maxItems. These keywords do not constrain the number of properties in an object, so the generated specification loses the Marshmallow length bounds.

Use minProperties/maxProperties for dictionaries and nested schemas that produce a single object. Determine nested cardinality from the resolved schema's many setting, including when it is inherited from a schema instance. Extend regression coverage for exact lengths, arrays, inherited many=True, and scalar Pluck fields, and add a changelog entry.

Validation:

• Six target regression cases fail before the fix.
• The full test suite passes: 648 passed, 6 skipped, with both the locked dependencies and the lowest supported Marshmallow 4.0.0.
• Applicable pre-commit checks pass, including Ruff, formatting, and mypy.
• A generated dictionary schema with length exactly one rejects empty and two-property objects under OpenAPI 2.0, 3.0, and 3.1.

Developed with OpenAI Codex using gpt-6.1-sol (xhigh); the patch and regression behavior were checked locally and independently reviewed before publication.

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