fix(http): cap standard retry delays

[thomhurst]

Summary

• cap every Standard HTTP retry delay at 10 seconds through RetryOptions.MaxDelay
• add HttpShield.RetryAfter(TimeSpan maxDelay) for custom server-suggestion caps without shortening a larger computed backoff
• document the Standard bound and Polly migration difference
• pin cap, backoff, validation, default, and pipeline-description behavior

The issue allows either a bounded delay or returning the response early. This uses the bounded-delay path because the current retry contract has no safe stop-and-retain-result sentinel; adding one would broaden into the retry-control work tracked separately.

Validation

• dotnet build Kevlar.slnx -c Release
• dotnet run --project tests/Kevlar.Tests -c Release --no-build -- --timeout 5m (850 passed)
• dotnet run --project tests/Kevlar.IntegrationTests -c Release --no-build -- --timeout 5m (131 passed)
• dotnet run --project tests/Kevlar.NetStandard.Tests -c Release --no-build -- --timeout 5m (10 passed)
• dotnet run --project tests/Kevlar.NetStandard21.Tests -c Release --no-build -- --timeout 5m (4 passed)
• pwsh scripts/Verify-Docs.ps1
• pwsh scripts/Verify-DocSnippets.ps1 -PackagesPath artifacts/package/release -Version 0.0.0-docs217 (119 snippets; behavior passed)
• npm ci --prefix docs
• npm run build --prefix docs

Depends on #258, whose repair commits are included in this branch.

Closes #217

[coderabbitai]

Important

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This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 51bc7304-ddf7-44aa-8a52-51e602293d61

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