refactor: Replace buildMethodConfig with clientMethods helper

- Add clientMethods() to filter and prepare method configs for client
- Remove buildMethodConfig(), getClientMethodConfig(), InferServiceTypes
- clientMethods() filters out external:false methods and strips server-only properties
- Types come from service class definitions, not runtime config
- Remove any types from method.ts, use proper typing
- Add custom methods type test to declarations.test.ts
- Update documentation for new pattern

Author: marshallswain