Add support for undefined partition keys in Cosmos DB

[Copilot]

• Add Undefined variant to InnerPartitionKeyValue enum
• Add public constant UNDEFINED to PartitionKey struct
• Update serialization in AsHeaders implementation to serialize undefined as {}
• Add single new test case undefined_value to validate undefined partition key
• Add assertion to existing tuple test to include undefined value
• Run clippy to ensure no linting errors
• Run cargo fmt to format code
• Update CHANGELOG.md with PR number and link

Original prompt

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This section details on the original issue you should resolve

<issue_title>Cosmos: Add a way to represent an undefined Partition Key</issue_title>
<issue_description>We currently support specifying a null value for a partition key in the Cosmos SDK, but have no way to express an undefined partition key. The undefined partition key identifies the partition containing items with no partition key property at all.

In the .NET SDK I believe we call this value None, but we should consider other names, since we already use EMPTY to refer to an empty set of partition keys when making a query (indicating a cross partition query)</issue_description>

<agent_instructions>* Add a marker for Undefined to InnerPartitionKeyValue

• Add a public constant UNDEFINED to PartitionKey that returns a PartitionKeyValue wrapping this marker
• Update the serialization code to serialize an undefined value as an empty JSON object: {}
• Do NOT write any From implementations, this is a niche option that will be specified directly
• Add a SINGLE new test case for undefined, and add a second assertion to the tuple test case that includes an undefined value.
• Ensure code compiles and tests pass
• Run cargo fmt when tests are passing
• Update CHANGELOG.md with a SINGLE line item for this change in the latest version, matching the format of other entries.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #2586

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[analogrelay]

@copilot update this with the PR number and link now that we have it.

[Copilot]

Updated the CHANGELOG entry with PR #3075 in commit 4eb4ac2.

[Copilot]

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