[DataProtection] Fix C# backup job duration format

[Minghao Chen (HarveyLink)]

Summary

Fixes Azure/azure-sdk-for-net#59573.

The REST contract remains unchanged: AzureBackupJob.duration is still a string and generated Swagger is unchanged. This client-only customization defines a DurationConstant scalar with duration-constant encoding and applies it only to the C# alternate type for AzureBackupJob.duration. A targeted known-encoding suppression documents why this service-specific encoding is required. The C# generator therefore emits constant-format ("c") TimeSpan parsing without changing other language SDKs.

Companion SDK draft PR: Azure/azure-sdk-for-net#63305

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API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
Python	azure-mgmt-dataprotection

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TypeSpec suppressions requiring review (testing, non-blocking)

Status: ❌ Approval required (currently under testing, review NOT enforced) — 1 suppression

⚠️ This check is currently in testing mode and is non-blocking — it will not prevent this PR from merging. This PR adds or updates the TypeSpec suppressions listed below. Suppressions are strongly discouraged — they bypass linter rules that protect API quality and consistency. Authors should avoid adding new suppressions and prefer fixing the underlying issue; reviewers should approve only when there is a clear, compelling justification and no reasonable alternative. Review each linked rule and source location, then apply Approved-TypeSpecSuppression only if every justification is acceptable. The Status column shows ✅ once the label is applied and ❌ while approval is pending.

New suppressions (1)

Status	Rule	Source	Justification
❌	@azure-tools/typespec-azure-core/known-encoding Check for supported encodings.	client.tsp#L8	The Data Protection service returns .NET constant-format durations, and this encoding is used only by the C# client alternate type.

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