Create a different OpenAPI Handler for each "tag"

### Motivation

The generator currently only creates one big OpenAPI Handler protocol.
This makes it hard to organize the routes.

### Proposed solution

Create a different OpenAPI Handler for each "tag" so we have something close to Vapor "controllers".
These are quite useful for decluttering the code.

The generator _could_ also generate each tag's "controller protocol" in it's own file. There are known issues in the Swift compiler where it struggles to handle one big file, or makes builds potentially extremely slower.

### Alternatives considered

Manually handling this by different methods, including manually creating controllers and redirecting the routes in the main openapi handler to those controllers, or using multiple targets and filtering a shared openapi file based on tag in each controller.

### Additional information

_No response_

The issue does not name specific files, tests, or generator entry points. Start by tracing how the current single OpenAPI Handler is generated and how tags are represented, then determine the expected per-tag protocols and file layout before defining completion checks.

Written by the indexing model from the issue text.