fix: Validate max value in ProgressTask to ensure it is zero or greater

[Rain6435]

Fixes #2167

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• 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

This change puts maxValue validation in ProgressTask at the exact points where invalid values can enter task state, so the invariant is enforced centrally instead of at call sites.

Constructor guard in ProgressTask.cs:142: validates initial maxValue when a task is created. This covers all AddTask overloads because they all flow into ProgressTask construction through ProgressContext internals.
Update-path guard in ProgressTask.cs:231: validates later changes via task.MaxValue setter, which routes through Update(maxValue: ...). This prevents a task from becoming invalid after successful creation.
Regression tests in ProgressTests.cs:142 and ProgressTests.cs:167: one test verifies creation-time rejection (ctx.AddTask(..., maxValue: -1)), and one verifies mutation-time rejection (task.MaxValue = -1), so both entry points are locked down.
Putting validation in these two locations ensures consistent runtime behavior for every API path without duplicating checks across ProgressContext overloads or user call sites.

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[Rain6435]

@microsoft-github-policy-service agree

[patriksvensson]

Thanks for your contribution!

A question: Wouldn't having a max value of -1 and a min value of -10 be a valid configuration?

[Rain6435]

@patriksvensson Good question. In ProgressTask we don’t have a MinValue concept, only Value and MaxValue, so a configuration like max=-1/min=-10 is outside the current model. Indeterminate progress is represented explicitly via IsIndeterminate, and completion is Value >= MaxValue, which means max=-1 would mark a new task as finished immediately. That’s why this change enforces MaxValue >= 0 and keeps indeterminate behavior on the dedicated IsIndeterminate path. Also, this is intuitive way of usage of the ProgressTask.