[feat] add openapi defaults handling for tests

[ldmonster]

Adds OpenAPI schema support to the functional testing framework so hook tests start from the same values state addon-operator produces in production.

What's included:

• LoadOpenAPISchema — reads an OpenAPI v3 YAML schema, resolves the x-extend parent chain (same semantics as addon-operator's ExtendTransformer)
• SchemaDefaults — walks a loaded schema and extracts a values document with all default: values applied recursively (objects, arrays, nested sub-properties)
• MergeValues — deep-merges two values maps with override-wins semantics (objects recursed, scalars/arrays replaced)
• Three new Options for NewHookExecutionConfig:
  • WithOpenAPIDir(dir) — loads <dir>/values.yaml and <dir>/config-values.yaml if present, silently ignores absent files
  • WithValuesSchema(path) / WithConfigValuesSchema(path) — fail-fast targeted variants
• Self-contained stub schemas in testing/framework/openapi_stub/ so framework tests don't depend on example-module files
• Three new framework tests in examples/example-module/hooks/subfolder/openapi_framework_test.go covering defaults, user overrides, and composing with an existing hook handler