fix: IsFinished should return false when a task has not started

[MateusMo]

Fixes #2182

• I have read the Contribution Guidelines
• I have checked that there isn't already another pull request that solves the above issue
• All newly added code is adequately covered by tests
• All existing tests are still running without errors

Changes

ProgressContext.IsFinished previously used _tasks.Where(x => x.IsStarted).All(task => task.IsFinished). When no task in the context had started yet (for example, tasks created with autoStart: false), Where(...) produced an empty sequence, and .All() on an empty sequence always returns true (vacuous truth). This caused IsFinished to report true even though no work had actually run, silently breaking the common while (!ctx.IsFinished) usage pattern.

• ProgressContext.cs: IsFinished now returns true only when there are no tasks in the context, or when every task's own IsFinished is true. Since ProgressTask.IsFinished is false for any task that hasn't reached its MaxValue (including one that hasn't started), this naturally treats "not started" as "not finished" without needing a separate IsStarted check.
• ProgressTests.cs: added four regression tests covering: no tasks in the context, a single task with autoStart: false, a task that has started and finished, and a mix of one finished task alongside one not-started task.

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[MateusMo]

@microsoft-github-policy-service agree

[patriksvensson]

Thanks!

Due to the very long "Changes" section I need to ask; there was no AI involved in this PR?

[MateusMo]

Yes, I found the behavior running a console app calling the method and used claude ai to help me to validate the scenarios and improve the test coverage

[MateusMo]

Any update on this one? Happy to make changes if needed