feat: add WithBodyDecoder option for custom JSON body decoding

[joncanning-rvu]

Summary

Adds a WithBodyDecoder router option that lets consumers customize how JSON request bodies are decoded, following the same functional options pattern as WithErrorHandler and WithMiddleware.

Motivation

The generated adapter calls runtime.DecodeJSONBody directly, with no way to intercept the decoding pipeline. When deserialization fails (e.g., null in a typed array, invalid date formats), the error handler receives a raw Go error with limited context.

A custom body decoder enables:

• Pre-processing request bodies before decoding (e.g., stripping nulls from arrays)
• Lenient decoding that tolerates schema mismatches the application wants to handle downstream
• Error enrichment with application-specific context (e.g., JSON pointer paths)

Changes

• pkg/runtime/json.go — Added BodyDecoderFunc type: func(body io.Reader, target any) error
• pkg/codegen/templates/handler/adapter.tmpl — Added bodyDecoder field to HTTPAdapter, defaults to runtime.DecodeJSONBody, calls a.bodyDecoder instead of hardcoded runtime.DecodeJSONBody
• All 13 router templates — Added bodyDecoder to routerConfig, added WithBodyDecoder option, wired to adapter after construction
• examples/server/test/server_test.go — Integration tests for the new option

The remaining generated file changes are from make generate.

Backward compatibility

• NewHTTPAdapter signature is unchanged — still (svc, errHandler)
• Default behavior is identical — runtime.DecodeJSONBody is used when no custom decoder is set
• WithBodyDecoder is additive — existing code that doesn't use it is unaffected

Usage

openapi.NewRouter(svc,
    openapi.WithBodyDecoder(func(body io.Reader, target any) error {
        // Custom logic: pre-process, decode, enrich errors
        return runtime.DecodeJSONBody(body, target)
    }),
)

Test plan

• All existing tests pass (make test)
• NewHTTPAdapter signature unchanged (no breaking change)
• All 13 router backends updated consistently
• Integration tests: custom decoder is invoked, body transformation works

[cubahno]

Other than renames, looks good, thank you!

[cubahno]

could u plz rename it to JSONBodyDecoderFunc and the With option too because it applies only to json bodies.