Assert what github_graph actually returns

[Thayorns]

test_simple_use and test_use_with_global_variables called Fbe.github_graph and stopped, never looking at what came back. Both stayed green whether the factory returned Fbe::Graph::Fake, a real client, or nil, so the one place where testing mode could accidentally reach the live API had no assertion guarding it.

test_simple_use now checks the fake is returned when testing is on, and a companion test covers the other branch of the same decision, where testing is not set and a real Fbe::Graph is built instead. A third checks that testing wins over a github_token when both are given, which is the combination a judge run actually hands over.

Three more cover the memoization the factory does through global[:github_graph]: the same object comes back on a second call inside one global, a second global gets its own object, and the same holds on the path that reads the global variables. That last one sets and clears $global, $options and $loog itself rather than leaning on test_use_with_global_variables, which now owns that cleanup.

Closes #665

@yegor256 please take a look

[Thayorns]

@yegor256 please take a look, happy to clarify anything.

[Thayorns]

@yegor256 fixed here, take a look please

[yegor256]

This PR now has merge conflicts and needs a rebase before it can go in. Assigning to @yegor256 for a look.

---

Generated by Claude Code

[Thayorns]

@yegor256 take a look please