[ricred]

Fixes #123

Root cause

With UseSafeMode on, the first query on a GraphQuery/GraphCursorQuery runs a one-time setup (InitQueryAsync): wait on a SemaphoreSlim, then fetch the schema via GraphClient.GetSchemaForSafeModeAsync() unless it is already in IMemoryCache. Neither step took the caller's CancellationToken, so cancelling ExecuteAsync/ExecuteWithResultAsync (directly or through the cursor pager) could still block indefinitely on the setup lock and on the introspection query.

Changes

• InitQueryAsync(CancellationToken): _lock.WaitAsync(ct) (acquire outside the try, so a cancelled wait cannot release a lock it never took) and GetSchemaForSafeModeAsync(ct). intialized is only set after the whole setup succeeded, so a cancelled init is simply retried by the next call.
• GraphClient.GetSchemaForSafeModeAsync(CancellationToken): the token flows into the introspection request. Safety of passing a per-caller token into the IMemoryCache.GetOrCreateAsync factory is documented on the method: GetOrCreateAsync is not single-flight (each cache miss runs its own factory) and a factory that throws - including on cancellation - commits nothing, so a cancelled fetch is never cached and other callers are unaffected.
• New token overloads GraphBaseExecute.GetRequestAsync(CancellationToken), GetRequestAsJsonAsync(CancellationToken) and GraphClient.GetSchemaForSafeModeAsync(CancellationToken); the existing parameterless methods are kept, so this is binary compatible. (Per .NET library guidance, adding a parameter to an existing method is a breaking change, hence overloads.)
• ExecuteBaseAsync/ExecuteBaseWithResultAsync pass their token to GetRequestAsync(ct); the cursor pager already forwards its token there.
• CHANGELOG entry under Unreleased > Fixed.

Not changed

• Linq2GraphQL.Client.Subscriptions: StartAsync()/StartWithErrorsAsync() accept no CancellationToken today, so the parameterless GetRequestAsync() call remains. A StartAsync(CancellationToken) overload would be a natural follow-up.

Testing

• 9 new tests in GraphBaseExecuteTests (in-process handlers, no live server): cancel while waiting for the setup lock (holder is not disturbed), cancel during introspection, pre-cancelled token sends no request, cancelled schema fetch is not cached and init is retried successfully, plus overload-compat coverage of the parameterless forms.
• Tests are hardened against the false pass this bug class invites: test clients use HttpClient.Timeout = Timeout.InfiniteTimeSpan (the 100 s default timeout produces a TaskCanceledException that would satisfy an OperationCanceledException assertion) and every bounded wait goes through Task.WaitAsync(10 s).
• Mutation-checked: dropping the token at each of the three injection points (schema fetch, lock wait, ExecuteBase* call) fails at least two of the new tests.
• dotnet build Linq2GraphQL.CI.slnf: 0 errors / 0 warnings. dotnet test Linq2GraphQL.CI.slnf: 454/454 passed. dotnet format --verify-no-changes on the touched files: clean.

[ricred]

@joadan could you take a review? This fixes your #123 (safe-mode setup ignoring the cancellation token).