[DataProtectionBackup] Fix constant-format backup job duration parsing

[Minghao Chen (HarveyLink)]

Summary

Fixes #59573.

AzureBackupJob.duration remains a REST string, but the C# client customization now maps it to a DurationConstant scalar encoded as duration-constant. The generated SDK consequently uses the .NET constant ("c") format for TimeSpan serialization/deserialization, accepting values such as 00:34:34.9902732 without changing the public API or Swagger contract.

This PR includes the generated serializer/deserializer fix, regression coverage, changelog entry, and updates tsp-location.yaml to companion REST-spec draft PR Azure/azure-rest-api-specs#46623 at commit 2f91f918ab7a264dc01bd01635c74e1f684d2ec8.

Testing

• dotnet test sdk\\dataprotection\\Azure.ResourceManager.DataProtectionBackup\\tests\\Azure.ResourceManager.DataProtectionBackup.Tests.csproj --filter FullyQualifiedName~DeserializeBackupJobWithConstantDuration

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Management SDK Review Summary

• Scope: sdk/dataprotection/Azure.ResourceManager.DataProtectionBackup
• Versioning: pass (CHANGELOG.md updated under Bugs Fixed; no ApiCompatVersion/major-version changes)
• API surface: pass — no changes to api/**; changed files are limited to generated serialization internals (src/Generated/Models/DataProtectionBackupJobProperties.Serialization.cs), tests/, CHANGELOG.md, and tsp-location.yaml
• Contextual naming: not applicable — no new public types introduced
• ApiCompat / breaking changes: pass — net - pullrequest CI check succeeded on head commit 85964ea9; no public API surface changed, so no breaking-change risk

No blocking management SDK review issues were found. The change updates the Duration TimeSpan format specifier from "P" to "c" in the generated serializer/deserializer to match a companion TypeSpec draft PR that encodes the property as duration-constant, adds a unit test covering constant-format deserialization, and bumps tsp-location.yaml to the draft spec commit. This is internal serialization behavior only; the public API shape (DataProtectionBackupJobProperties.Duration as TimeSpan?) is unchanged.

Note: tsp-location.yaml currently points to a draft/unmerged spec PR (Azure/azure-rest-api-specs#46623) rather than a merged main commit — this should be updated to a merged spec commit before this SDK PR is finalized, per the standard TypeSpec-backed package workflow.

Analyzed by Azure .NET Management SDK PR Review: https://github.com/Azure/azure-sdk-for-net/actions/runs/35840829115