feat: add BigQueryMockTable class for explicit three-part naming

[gurmeetsaran]

Summary

Introduces BigQueryMockTable class to provide clean, explicit support for BigQuery's three-part naming scheme (project.dataset.table).

Previously, BigQuery mock tables had to awkwardly cram project and dataset together into the database_name field. The new class makes each component explicit using simple class variables.

Changes

Implementation

• New BigQueryMockTable class in src/sql_testing_library/_mock_table.py
  • Three mandatory class variables: bigquery_project, bigquery_dataset, bigquery_table
  • Supports inheritance pattern for code reuse
  • Backwards compatible with BaseMockTable interface
  • No instance variables, no global state, no complex logic

Tests

• 16 comprehensive unit tests in tests/test_bigquery_mock_table.py
• Updated integration tests to use new class in tests/integration/test_bigquery_integration.py
• All existing tests continue to pass

Documentation

• Updated README.md with BigQueryMockTable examples and benefits
• Updated docs/adapters.md with detailed BigQuery-specific section
• Updated docs/api-reference.md with complete API documentation
• Updated docs/examples.md with practical usage examples
• Removed examples/ folder files (redundant with inline documentation)
• Fixed broken documentation references

Example Usage

Before (Awkward)

class UsersMockTable(BaseMockTable):
    def get_database_name(self) -> str:
        return "test-project.test_dataset"  # Confusing!
    
    def get_table_name(self) -> str:
        return "users"

After (Clean)

class UsersMockTable(BigQueryMockTable):
    bigquery_project = "test-project"
    bigquery_dataset = "test_dataset"
    bigquery_table = "users"

Inheritance Pattern

# Base class with shared project
class MyProjectTable(BigQueryMockTable):
    bigquery_project = "my-project"

# Subclasses just set dataset and table
class UsersTable(MyProjectTable):
    bigquery_dataset = "analytics"
    bigquery_table = "users"

Benefits

• ✅ Clear Semantics - Each BigQuery component is explicit
• ✅ Simple - Just 3 mandatory class variables
• ✅ Type Safe - Full type hints and IDE autocomplete
• ✅ No Complexity - No instance variables, global state, or priority logic
• ✅ Backwards Compatible - Still implements all BaseMockTable methods
• ✅ Flexible - Use inheritance to share common properties

Test Results

• ✅ All 16 BigQueryMockTable unit tests pass
• ✅ All 19 existing BaseMockTable tests pass
• ✅ All linting and type checks pass
• ✅ Zero breaking changes