test(vision): add modality contract fixtures and checks

[a-klos]

This pull request introduces contract tests to ensure the consistency and validity of modality metadata across different extraction paths. It adds a new test suite that validates sample fixtures for various modalities (TEXT, TABLE, IMAGE), including both valid and invalid cases, and enforces schema requirements for each. The most important changes are as follows:

Test Suite Implementation:

• Added a new contract test module (modality_contract_test.py) that loads JSON fixtures and validates that modality metadata conforms to required schemas, including special handling for IMAGE modality and checks for uniqueness of fixture names.

Test Fixtures for Modality Validation:

• Added a valid TEXT modality fixture (text_valid.json) to verify correct schema for text pieces.
• Added a valid TABLE modality fixture (table_valid.json) to test schema compliance for table pieces.

Test Fixtures for IMAGE Modality:

• Added a valid IMAGE modality fixture using legacy base64 encoding (image_legacy_base64_valid.json).
• Added a valid IMAGE modality fixture using an image reference and MIME type (image_reference_valid.json).
• Added an invalid IMAGE modality fixture missing required payload fields to confirm negative case handling (image_invalid_missing_payload.json).