.NET: add AgentSession StateBag edge case coverage

[challaravinath]

Motivation and Context

AgentSession.StateBag had only one existing test covering a basic set/get roundtrip.
Edge cases such as empty initial state, independent multi-key storage, and value overwrite behavior were not tested through the AgentSession layer, even though they are critical for session state correctness.

Description

• Add test to verify AgentSession.StateBag is empty on initialization
• Add test to verify multiple keys store and retrieve independently without interference
• Add test to verify overwriting an existing key returns the updated value

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• dotnet test — 1141 tests, 0 failed
• Is this a breaking change? No — this PR only adds new tests, no production code was modified.

[Copilot]

Pull request overview

Adds three unit tests to cover previously untested edge cases of AgentSession.StateBag: default-empty state, independent multi-key storage, and value overwriting.

Changes:

• Add StateBag_Default_IsEmpty test
• Add StateBag_MultipleKeys_StoreAndRetrieveIndependently test
• Add StateBag_OverwriteValue_ReturnsUpdatedValue test

[challaravinath]

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