Terminal Chat: Add support for custom AI providers like openrouter

[inlineHamed]

Description of the new feature

Current list of providers has only three options which is limiting. It would be much better if we can other oprions like openrouter.ai vs code and powertoys already support that i tried the same way of powertoys as said here (https://github.com/microsoft/PowerToys/issues/32960#issuecomment-3591867556) using azure ai option but was not worked here

Proposed technical implementation details

No response