feat(ai-promp-guard) add config option to prompt guard for scanning "system" and "assistant" messages

[fffonion]

Summary

This PR also fixes an issue when allow_all_conversation_history
is set to false, the first user request is selected instead of the last
one.

Unit tests are refactored to so that it's cleaner and has wider coverage.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

AG-58

[fffonion]

@jschmid1 could you take another look on this? Thanks!

[jschmid1]

If you add a test for backwards compat verification, we can move this forward. In the meanwhile, I'll request another reviewer @Kong/gateway-moderators

[Tieske]

This line shouldn't be removed, it breaks the docs format

[fffonion]

there's another treturn[2] below, this is a typo i think

[Tieske]

each line is a (typed) value, the numbers indicate the return "options";

1. a valid result, 1 value, being a table
2. an error which returns 2 values:
   • a nil value
   • an error message

so 3 lines in total

see first example here: https://lunarmodules.github.io/ldoc/examples/multiple.lua.html

[fffonion]

I see, in other places however we use following patterns:

-- @treturn table          something
-- @treturn string|nil    error

should we align with that instead?

[fffonion]

updated with @treturn string|nil

[Tieske]

no, that breaks it even more. The original was correct. Not sure where you found those other references, but they are also wrong most likely.

[fffonion]

There's PDK and other plugins uses this pattern, a took a snippet below:

kong/db/schema/topological_sort.lua:-- @treturn nil|string nil if the schemas were sorted, or a message if a cycle was found
kong/db/declarative/init.lua:-- @treturn nil|string error message, only if error happened
kong/db/declarative/init.lua:-- @treturn nil|table err_t, only if error happened
kong/db/declarative/init.lua:-- @treturn nil|string error message, only if error happened
kong/db/declarative/init.lua:-- @treturn nil|table err_t, only if error happened
kong/db/declarative/init.lua:-- @treturn nil|string error message if error
kong/db/declarative/init.lua:-- @treturn nil|table err_t if error
kong/tools/stream_api.lua:-- @treturn nil|string error
kong/tools/stream_api.lua:-- @treturn nil|string error   an error string
kong/tools/stream_api.lua:-- @treturn nil|string error  an error string, returned for protocol or socket I/O failures
kong/tools/stream_api.lua:-- @treturn nil|string   error
kong/tools/mime_type.lua:-- @treturn nil|nil|nil Invalid mime-type
kong/workspaces/counters.lua:-- @treturn nil|string error
kong/openid-connect/claims.lua:-- @return status(nil|boolean), error(string)
kong/openid-connect/claims.lua:-- @return status(nil|boolean), error(string)
kong/openid-connect/claims.lua:-- @return status(nil|boolean), error(string)
kong/openid-connect/claims.lua:-- @return status(nil|boolean), error(string)
kong/pdk/client/tls.lua:  -- @treturn nil|err Returns `nil` if successful, or an error message if it fails.
kong/pdk/client/tls.lua:  -- @treturn nil|err Returns `nil` if successful, or an error message if it fails.
kong/pdk/client/tls.lua:  -- @treturn nil|err Returns `nil` if successful, or an error message if it fails.
kong/pdk/request.lua:  -- @treturn nil|string An error message.
kong/pdk/client.lua:  -- @treturn nil|err `nil` if successful, or an error message if it fails.
kong/pdk/client.lua:  -- @treturn nil|err `nil` if successful, or an error message if it fails.

Right now, only the newly added ai-prompt-guard and ai-azure-content-safety plugin uses the new pattern. I'm not sure how we have two different patterns and our docs are rendering fine with the PDK ldoc.
Anyway, I'm going to just revert this change. I do not use ldoc locally so I don't have clue how to verify it.

[Tieske]

I don't know who wrote that, but most of that is wrong.

[fffonion]

would you take another look at this PR? @Tieske

[jschmid1]

👍🏼