perf(validation): optimize overlap checks

[pavelnikolov]

• Group fields by response key to cut overlap comparisons (addresses Control memory explosion on large list of queries #102).
• Add identical-field shallow+hash fast path (k >= 3) with per-op hash cache.
• Add benchmarks and a //go:build bench tag to conditionally build benchmarks.
• Minor refactors: simplify allIdenticalFields & shallowFieldEqual.

[Copilot]

Pull Request Overview

This PR optimizes overlap validation in GraphQL queries by implementing performance improvements to reduce O(n²) comparison complexity. The optimization focuses on grouping fields by response key and adding a fast path for structurally identical fields.

• Groups fields by response key to avoid unnecessary overlap comparisons between fields with different aliases
• Adds an identical-field fast path with structural hashing for groups of 3+ fields
• Includes comprehensive benchmarks to measure performance improvements

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/validation/validation.go	Core optimization implementation with field grouping, hash-based identical field detection, and structural comparison functions
internal/validation/pathological_overlap_bench_test.go	Benchmark testing pathological overlap scenarios with alternating field definitions
fragment_queries_bench_test.go	Comprehensive benchmarks for fragment queries and alias scenarios to measure validation performance

Comments suppressed due to low confidence (1)

fragment_queries_bench_test.go:55

• The b.Loop() method does not exist in the standard testing package. This should be for i := 0; i < b.N; i++ or use for range b.N in Go 1.22+.

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