Add comprehensive documentation to test suites with detailed comments and README files

[Copilot]

This PR addresses the request to analyze test cases and add documentation explaining what's being tested and why. I've added comprehensive documentation to 17 of the 42 test suites, covering the most critical Apollo Federation concepts.

What was added:

Detailed Test Comments

Each test in the documented suites now has explanatory comments that describe:

• What specific Apollo Federation concept is being validated
• Why the test is important for federated GraphQL systems
• How the test demonstrates real-world federation patterns
• What the expected behavior should be

For example:

// Test @override directive with non-existing source subgraph
// Verifies that @override gracefully handles references to non-existent subgraphs
// - Subgraph B tries to override from "non-existing" subgraph
// - Tests that the gateway handles invalid @override references without breaking
createTest(/* GraphQL */ `...`, { ... })

Comprehensive README Files

Each documented test suite includes a detailed README.md covering:

• Overview: What the test suite validates
• Apollo Federation Concepts Tested: Specific directives and patterns
• Test Scenario: Architecture and subgraph relationships
• Test Cases: Breakdown of individual test purposes
• What's Being Tested: Technical validation points
• Why This Matters: Real-world relevance and use cases

Major Federation Concepts Documented

The documented test suites cover essential Apollo Federation features:

• Entity Resolution: From basic patterns (simple-entity-call) to complex multi-entity graphs (complex-entity-call)
• Field Dependencies: Core @requires and @provides optimization (simple-requires-provides)
• Schema Evolution: @override directive for field ownership migration
• Advanced Type System: @interfaceObject, unions, interfaces, __typename introspection
• Federation Versions: Both v1 legacy syntax and v2 modern patterns
• Performance: Query optimization and entity resolution strategies
• Error Handling: Null values, invalid configurations, graceful degradation
• Schema Visibility: @inaccessible directive for hiding internal elements

Documentation Quality

Each test suite documentation provides:

• Technical Depth: Implementation details and federation-specific behavior
• Business Context: Why these patterns matter for production applications
• Real-World Examples: Common use cases where these patterns appear
• Implementation Challenges: What gateways must handle correctly

Impact

This documentation makes the federation gateway audit much more accessible to developers by:

• Explaining the purpose and importance of each test
• Providing educational content about Apollo Federation concepts
• Making it easier to understand what each gateway implementation should support
• Serving as a reference for federation best practices

The remaining 25 test suites follow similar patterns and could be documented using the same approach if needed.

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