feat: allow priority override when multiple SchemaGeneratorHooksProviders are registered

[gaurav0107]

📝 Description

Adds a backward-compatible priority() default method to the SchemaGeneratorHooksProvider SPI (returns 0 by default) and teaches both generateSDL and generateGraalVmReflectMetadata to pick the provider with the strictly highest priority when more than one provider is discovered through ServiceLoader.

Behaviour preserved for existing users:

• Zero providers still warn and fall back to NoopSchemaGeneratorHooks.
• Exactly one provider still wins unchanged.
• Multiple providers at the same priority still fail fast. The error message keeps the legacy "Cannot generate SDL as multiple SchemaGeneratorHooksProviders were found on the classpath" prefix but now also lists the tied class names and their priority so the conflict can be diagnosed without shade-plugin hacks.

Opt-in path for library authors. A library that ships a SchemaGeneratorHooksProvider which must override a transitively-imported one can now override priority() to return a positive value instead of telling consumers to exclude the upstream library.

Scope.

• SPI: adds a default method to SchemaGeneratorHooksProvider (source- and binary-compatible).
• Call sites: replaces the inline when block in GenerateSDL.kt and GenerateGraalVmMetadata.kt with a small package-local selectHooksProvider(providers, logger) helper in each module.
• Tests: adds unit tests for the new selection helper in both modules (single-provider passthrough, highest-priority wins, same-priority tie throws with descriptive message).

A shared extraction into graphql-kotlin-hooks-provider felt out of scope for this change — both helpers are tiny and duplicating them keeps the SPI module dependency-free on SLF4J. Happy to hoist them if maintainers prefer.

🔗 Related Issues

Closes #2150