Loop detection

Add logic to detect when an API might be calling itself (directly or via another API) in a loop, and terminate the loop.

This can be done through injection of unique headers for every hop through the gateway.

For example:

Client request -> GW: `GET /api1`
GW inject header: `X-ApiGw-Loop: get_api1`
GW request -> GW: `GET /api2`
GW inject header: `X-ApiGw-Loop: get_api1 get_api2`
GW request -> GW: `GET /api1`
GW analyzes `X-ApiGw-Loop` and sees `get_api1` is already present
GW response: `508 Loop detected` (or similar)

Start by tracing how the gateway forwards requests between APIs and how it handles injected headers. Define how the hop header records each API and how repeated entries produce a loop response such as 508. Done means direct and indirect API loops terminate while non-looping requests continue normally.

Written by the indexing model from the issue text.