Add tests for Xai provider

[sambradbury]

Adds some tests requested by @salman1993 here: block#2976 (comment)

 ~/Documents/code/goose/ [add-xai-tests] cargo test test_xai_provider --test providers
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.25s
     Running tests/providers.rs (target/debug/deps/providers-a220957366eecf4c)

running 1 test
test test_xai_provider ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 13 filtered out; finished in 0.00s


============== Providers ==============
⏭️ Xai
=======================================

 ~/Documents/code/goose/ [add-xai-tests] cargo test test_agent_with_xai --test agent  
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.24s
     Running tests/agent.rs (target/debug/deps/agent-da2b34cd632a2304)

running 1 test
test tests::test_agent_with_xai ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 12 filtered out; finished in 0.00s