feat(cosmos): support hierarchical partition keys

[ScottArbeit]

Closes #9899

Problem

Cosmos grain storage models each record with a single string partition key, so applications cannot use Cosmos DB hierarchical partition keys for tenant-aware distribution without replacing the storage provider.

Solution

• Add an ordered document-key contract that supports one, two, or three string partition-key components while preserving existing IDocumentIdProvider implementations.
• Persist the selected components in fixed fields and use the same resolved values for Cosmos point operations.
• Validate configured partition-key paths against existing containers, including legacy /GrainType compatibility for the built-in provider.
• Document HPK configuration with compiled snippets and focused compatibility coverage.

Rationale

Keeping document identity and ordered partition-key values together prevents request keys from drifting from persisted fields. The level count makes HPK opt-in, preserves existing single-key behavior, and rejects mismatched container definitions during startup.

Microsoft Reviewers: Open in CodeFlow

[ScottArbeit]

Sorry, didn't mean to dump this work-in-progress here yet. 😳 I'm going to close it for now and check in about doing this correctly, I don't want to be rude.