Centralize import/helper tracking via CodegenContext and add behavior…

[mromaszewicz]

…al tests for helper templates

Replace the scattered ParamUsageTracker and per-phase import collection with a single CodegenContext that all generators share. This eliminates duplicate param tracker creation across client, server, webhook, and callback code paths, and moves import registration, helper tracking, param function tracking, and custom type tracking into one place.

Key changes to code generation pipeline:

• Add CodegenContext (codegen_context.go) as the centralized tracker for imports, helpers, param functions, and custom type templates
• Refactor Generate() in codegen.go into three phases: (1) generate all code sections, (2) render helpers/params/custom types, (3) assemble output — eliminating repeated param/form-helper generation per phase
• Remove ParamUsageTracker from paramgen.go; its logic is absorbed by CodegenContext.NeedParam(), GetRequiredParamTemplates(), etc.
• Update TypeGenerator to delegate imports and template tracking to ctx
• Update GatherOperations, GatherWebhookOperations, and GatherCallbackOperations to accept *CodegenContext instead of *ParamUsageTracker
• Move loadCustomType into CodegenContext.loadAndRegisterCustomType so custom type imports are registered directly on the context

Add behavioral tests for helper function templates (test/helpers/):

• genhelpers/main.go: CLI generator tool that reads ParamHelpersTemplate, MarshalFormHelperTemplate, and ParamTemplates registries, writes .gen.go files with goimports to prune unused imports
• 16 generated .gen.go files: helpers, marshal_form, plus style/bind for simple, simple_explode, form, form_explode, label, label_explode, and deepObject
• 7 test files (55 tests) covering marshalForm, primitiveToString, BindStringToObject, Date, escape/unescape, and roundtrip style-then-bind for all parameter styles