feat(credentials): add referenced credential element management (#7850)

[Gabriel Pezé (gabriel-peze)]

Proposed changes

• Add a centralized way to create credential reference element in the injectors

Testing Instructions

1. Step-by-step how to test
2. Environment or config notes

Related issues

• Related #7850

[Copilot]

🟡 Changes recommended

Address the missing multiple=False assertion and clarify the helper docstring.

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Pull request overview

Adds centralized credential-reference types, contract support, and provider mapping helpers for injector contracts.

Changes:

• Adds credential type enums and public exports.
• Adds credential-reference contract models and serialization.
• Adds provider mapping utilities and tests.

File summaries

File	Summary
test/credential/test_credential_utils.py	Tests credential mapping helpers.
test/credential/test_credential_types.py	Tests credential type enums.
test/credential/__init__.py	Adds credential test package support.
test/contracts/test_contract_referenced_credential.py	Tests contract serialization and defaults.
pyoaev/credential/utils.py	Provides provider mapping and field creation helpers.
pyoaev/credential/types.py	Defines credential reference types.
pyoaev/credential/__init__.py	Exposes the credential API.
pyoaev/contracts/contract_config.py	Adds credential-reference contract support.

Review details

• Files reviewed: 7/8 changed files
• Comments generated: 2
• Review effort level: Lite

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