Instead of hardcoding each router's framework and strict middleware
imports directly in imports.tmpl with per-router conditional blocks,
compute them in Go code via GenerateOptions.RouterImports() and pass
them to the template as RouterImports. The template now uses a single
range loop over .RouterImports, making it straightforward to add new
routers without touching the template.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Echo v5 version of the petstore-expanded example, in its own Go module
(go 1.25.0) to avoid bumping the Go version of the shared examples module.

Key differences from the echo (v4) example:
- *echo.Context (pointer) instead of echo.Context (interface)
- echo.NewHTTPError(code, message) takes exactly two args
- No HTTPError.Internal field
- EchoRouter returns echo.RouteInfo instead of *echo.Route
- log.Fatal(e.Start(...)) instead of e.Logger.Fatal(...)

Includes an inline OpenAPI request validation middleware (middleware/)
adapted from echo-middleware for echo v5, since no published echo-v5
middleware package exists yet.

Models and server stubs are placeholders until the codegen circular
dependency for go:generate in isolated modules is resolved.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>