Add WorkflowRetryPolicy example project

[Copilot]

Description

Adds examples/Workflow/WorkflowRetryPolicy — a self-contained example demonstrating WorkflowRetryPolicy usage on activity and child workflow calls, as described in the Dapr workflow retry policies docs.

Three retry scenarios are covered:

• Fixed-interval retry — activity retried up to 5 times with constant 1s delay
• Exponential back-off retry — activity retried with 2× backoff coefficient, capped at 16s
• Child workflow retry — sub-workflow retried via ChildWorkflowTaskOptions with 1.5× backoff

A FlakyActivity and FlakyChildWorkflow simulate transient failures by throwing for a configurable number of attempts before succeeding.

var options = new WorkflowTaskOptions(
    new WorkflowRetryPolicy(
        maxNumberOfAttempts: 10,
        firstRetryInterval: TimeSpan.FromSeconds(1),
        backoffCoefficient: 2.0,
        maxRetryInterval: TimeSpan.FromSeconds(16)));

await context.CallActivityAsync<string>(nameof(FlakyActivity), input, options);

Project added to all.sln under the examples/Workflow solution folder.

Issue reference

This PR adds the example project requested in the linked issue. #1500

Checklist

• Code compiles correctly
• Created/updated tests
• [] Extended the documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.37%. Comparing base (1f1fe91) to head (99ad312).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1789   +/-   ##
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  Coverage   62.37%   62.37%           
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  Files         285      285           
  Lines        8421     8421           
  Branches      980      980           
=======================================
  Hits         5253     5253           
- Misses       2934     2935    +1     
+ Partials      234      233    -1     

Flag	Coverage Δ
net10.0	62.36% <ø> (ø)

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[WhitWaldo]

Looks great - thank you!