fix: freeze ClientMetadata to satisfy spec req 4.2.2.1

[cristiangmarta]

Spec requirement 4.2.2.1 states that client_metadata in HookContext MUST be immutable.

ClientMetadata was an unfrozen Struct — any hook could mutate it, corrupting the client's @metadata for all subsequent evaluations since the same object is shared across hook invocations.

Adds .freeze at construction, consistent with how both built-in providers already handle ProviderMetadata.

[Copilot]

Pull request overview

Ensures client_metadata exposed by OpenFeature::SDK::Client is immutable to satisfy OpenFeature spec requirement 4.2.2.1 and prevent hooks from mutating shared client metadata across evaluations.

Changes:

• Freeze ClientMetadata when constructing a Client.
• Add a spec asserting client.metadata is frozen.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
spec/open_feature/sdk/client_spec.rb	Adds coverage asserting client metadata immutability via be_frozen.
lib/open_feature/sdk/client.rb	Freezes the ClientMetadata instance at client initialization.

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[gemini-code-assist]

Code Review

This pull request implements immutability for client metadata by freezing the ClientMetadata object during initialization and adding a corresponding test case. The review feedback points out that while the metadata object itself is frozen, the underlying domain string remains mutable, which could violate the immutability requirements. It is recommended to freeze the domain string as well and update the test suite to verify this deeper level of immutability.