Increase tests coverage.

Coverage Analysis

Overall coverage: 80.8% (5,326 covered / 6,588 statements, 1,262 uncovered)

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Files with highest impact on overall coverage (excluding test/bench/examples)

File	Coverage	Uncovered	Total Stmts	Impact if 100%
language/ast/type_definitions.go	0.0%	91	91	+1.38pp
language/ast/values.go	0.0%	57	57	+0.87pp
plan_cache_normalize.go	80.1%	44	221	+0.67pp
language/ast/definitions.go	0.0%	43	43	+0.65pp
language/visitor/visitor.go	90.6%	29	310	+0.44pp
plan.go	92.8%	27	373	+0.41pp
language/ast/selections.go	0.0%	25	25	+0.38pp
language/ast/types.go	0.0%	19	19	+0.29pp
schema.go	92.3%	18	235	+0.27pp
plan_cache.go	81.1%	18	95	+0.27pp
language/location/location.go	0.0%	16	16	+0.24pp
rules.go	97.8%	16	730	+0.24pp
values.go	92.7%	14	191	+0.21pp
subscription.go	84.5%	11	71	+0.17pp
rules_overlapping_fields_can_be_merged.go	96.7%	9	269	+0.14pp
util.go	89.7%	9	87	+0.14pp
scalars.go	97.1%	8	273	+0.12pp
language/ast/name.go	0.0%	6	6	+0.09pp
language/ast/arguments.go	0.0%	6	6	+0.09pp
type_info.go	96.1%	5	128	+0.08pp
validator.go	95.4%	5	108	+0.08pp
language/source/source.go	0.0%	5	5	+0.08pp
language/ast/document.go	0.0%	5	5	+0.08pp
language/ast/directives.go	0.0%	5	5	+0.08pp
language/ast/location.go	0.0%	3	3	+0.05pp
graphql.go	88.9%	3	27	+0.05pp
located.go	94.4%	1	18	+0.02pp
language/lexer/lexer.go	99.6%	1	282	+0.02pp

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Key takeaways

Best targets for real logic (not just type definitions)

1. plan_cache_normalize.go — 80.1% coverage, 44 uncovered. Biggest code-level gap with actual logic.
2. plan.go — 92.8% coverage, 27 uncovered. High statement count, core execution logic.
3. language/visitor/visitor.go — 90.6% coverage, 29 uncovered. Core AST traversal.
4. schema.go — 92.3% coverage, 18 uncovered. Schema building logic.
5. plan_cache.go — 81.1% coverage, 18 uncovered. Caching logic.
6. subscription.go — 84.5% coverage, 11 uncovered. Subscription execution.
7. util.go — 89.7% coverage, 9 uncovered. Utility functions.
8. values.go — 92.7% coverage, 14 uncovered. Value coercion/resolution.

These files contain actual logic and edge cases that aren't covered by existing tests.

Bulk coverage wins (language/ast/*)

The language/ast/ files (type_definitions, values, definitions, selections, types, etc.) collectively account for ~255 uncovered statements at 0% coverage (~20% of all uncovered code). These are primarily struct/type definitions used across the codebase. They would give the biggest numerical boost (+3.7pp total) but require adding AST package-level tests that exercise these types directly.