feat: Add x-codemode extension for OpenAPI spec

[thomaswinkler]

Summary

Introduces x-codemode, an OpenAPI extension to allow providing additional instructions or include additional operations into a code mode response. The DTM spec includeas a simple example for testing (e.g. PropertyDefinitions as prerequisite for AssetDefinition).

Each x-codemode item on an operation carries an instruction for the LLM and an optional path reference (include or query) that controls how much context is pre-loaded:

Field	Behavior
instruction	Pure LLM-only context — idempotency hints, required headers, constraints not visible in standard tooling
include	Mandatory prerequisite — the referenced path's full spec is embedded inline at build/discovery time. The LLM gets everything it needs without an extra query call.
query	Advisory hint — only the service-prefixed path (queryPath) is computed. The LLM queries it if and only if the user's request actually requires the dependency.

Notes

This is meant as an extension to improve modelling of agent experience within the OpenAPI. It depends on the use case and spec itself of when to include information into the user doc or as a code-mode extension only for the agent. Needs some more thoughts and use case evaluation. Results have been much better with DTM example extension for creating Asset Definitions including Property Definitions.