ref(logs): extract attribute helpers for metrics

[jpnurmi]

Summary

Extracts attribute helper functions from the logs implementation to make them reusable for the upcoming metrics feature.

Changes:

• Extract sentry__value_add_attribute() to sentry_value.c for adding typed attributes to telemetry data
• Extract sentry__scope_apply_attributes() to sentry_scope.c for applying user/OS/trace context
• Add sentry__apply_attributes() to sentry_core.c as the main entry point combining scope and options data

This is a pure refactoring with no behavioral changes - the logs feature continues to work exactly as before.

Test plan

• Existing logs integration tests pass
• No new public API changes

Relates to #1453

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