chore: declare per-context summary events support in client SDK metadata

[kinyoklion]

Overview

Follow-up to #288 (which enables per-context summary events in the .NET client SDK). This records that support in .sdk_metadata.json so sdk-meta's feature data reflects it.

• Adds a features object to the dotnet-client-sdk entry with perContextSummaryEvents (the canonical feature key from sdk-meta's feature_info.json — "Per-Context Summary Events"), introduced: "5.9".
• The client entry had no features section previously; this adds it with this one feature.

Notes

• introduced: "5.9" is the anticipated next client release (current is 5.8.0; feat: enable per-context summary events #288 is a feat, so release-please will cut 5.9.0). If the release lands at a different version, update this value. The metadata becomes accurate once 5.9 ships.
• Root-level metadata change only — not under a release-please package path, so it doesn't trigger an SDK version bump.

Relates to SDK-2450. Should land after #288 (the feature it documents).

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Note

Low Risk
Root-level SDK metadata only; no application, auth, or release logic changes.

Overview
Documents per-context summary events for the .NET Client SDK in .sdk_metadata.json by adding a new features block on dotnet-client-sdk with perContextSummaryEvents and introduced: "5.9", aligning sdk-meta with the client implementation from the related feature work.

This is metadata-only (no runtime or package code changes) and does not drive a release-please version bump.

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