Bugfix/self route toggle

[saisakul]

Context

In Redbox graph, when a user selects a document, there are 3 possible scenarios:
exceed limit (>1M) → return document is too large
exceed context (>128K) → self_route
pass (<=128K) go to summarise
In the current setting, self_route is off, which means summarise is always called, and search graph is never called.
This affects Redbox performance when users ask questions on specific parts of the document.

Before:

Changes proposed in this pull request

• create ENV var for setting self_route and change the code to get the setting from ENV
• Change the graph so that self_route is called when request token less than limit

After:

Guidance to review

Relevant links

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests