Refactor schema system to support provider-specific constraints

[aksg87]

Description

Refactors the schema constraint system to enable provider-specific implementations through a plugin architecture. Each LLM provider can now define its own optimal approach for structured output generation while maintaining backward compatibility.

Related to #128, #59, #99

Feature

How Has This Been Tested?

$ pytest tests/extract_schema_integration_test.py -v
$ pytest tests/factory_schema_test.py -v
$ pytest tests/provider_schema_test.py -v
$ tox -e live-api
$ tox -e ollama-integration

Verified backward compatibility with existing providers and tested schema generation with multiple providers.

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran pylint over the affected code.

Key Changes

Schema Abstraction (langextract/schema.py)

• BaseSchema abstract class for provider-specific implementations
• FormatModeSchema for JSON/YAML providers (OpenAI, Ollama)
• Property-based fence output detection

Factory Integration (langextract/factory.py)

• Enhanced create_model() with schema constraint support
• Automatic schema selection based on provider capabilities
• Full backward compatibility maintained

Provider Schemas (langextract/providers/schemas/)

• Moved GeminiSchema to dedicated module
• Clear separation between provider implementations

Testing

• E2E tests for schema functionality
• Provider-specific schema validation
• All existing tests pass without modification

Breaking Changes

None - full backward compatibility maintained. The refactoring is entirely internal with no changes to public APIs.