fix(core): migrate deprecated graphql-js APIs for v17/v18 compat

[babbarankit]

Summary

Closes #1637

Fixes enum and custom scalar default values breaking in graphql v17+. The root cause: graphql v17 validates default values using getDefaultValueAST() which requires the default: { value } API (external values), not just the deprecated defaultValue (internal values).

Key insight — the value-shape trap

astFromValue(value, type) takes internal/coerced values (e.g., 2 for enum TWO).
valueToLiteral(value, type) takes external values (e.g., "TWO").
Naive swap silently corrupts enum and custom scalar defaults.

Approach: serialize internal→external at build time

Rather than swapping astFromValue → valueToLiteral, we:

1. Serialize internal values to external form via type.serialize() at build time
2. Emit default: { value: serialized } alongside deprecated defaultValue
3. Prefer stored AST literals from v17+ default API in mock-ast, falling back to astFromValue (v16/v17) then valueToLiteral + serialize (v18+)

Files changed

File	Change
core/src/build-cache.ts	Emit default: { value: serializeDefaultValue(...) } in buildInputFields()
plugin-directives/src/mock-ast.ts	Replace 3x astFromValue() with internalValueToLiteral() helper
plugin-add-graphql/src/schema-builder.ts	Read from .default?.value ?? .defaultValue
plugin-sub-graph/src/index.ts	Copy default property in config reconstruction
converter/src/index.ts	Read from .default?.value ?? .defaultValue

Backward compat

All changes are additive — graphql v16 ignores unknown config keys, v17+ uses them. No breaking changes.

Test plan

• 155 tests pass across 8 packages (core, directives, prisma, sub-graph, add-graphql, relay, federation, converter)
• Fixes 2 pre-existing test failures in plugin-directives (enum default values in printSchema and validateSchema)
• New test: validates printSchema produces enumWithDefault: EN = TWO for enum with internal value 2
• Existing test validates AST structure for enum/boolean/string/ID/list defaults

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