Refactor Timeline Serialization to Use Generic ISerializer<T>

[rian-be]

Problem

Right now, MessagePackTimelineSerializer is Timeline specific serializer that duplicates MessagePack configuration, options, and ULID handling. This causes:

• repeated configuration code across serializers
• inconsistent serialization patterns across domain types
• harder testing and extending for new types

At same time, ISerializer<T> already provides generic, reusable serializer abstraction with MessagePack support and ULID handling.

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Goal

Replace MessagePackTimelineSerializer with generic solution using ISerializer<T>:

• Use ISerializer<Timeline> as the underlying serializer
• Make ITimelineSerializer thin adapter delegating to ISerializer<Timeline>
• Remove duplicate configuration for MessagePack options and ULID formatter

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Proposed Tasks

• Remove current MessagePackTimelineSerializer
• Implement TimelineSerializerAdapter : ITimelineSerializer that wraps ISerializer<Timeline>
• Register ISerializer<Timeline> in DI using MessagePackSerializer<Timeline>
• Update all consumers of ITimelineSerializer to use adapter
• Verify Timeline serialization/deserialization still works correctly

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Benefits

• Eliminates duplicate serializer configuration
• Provides single, reusable serialization mechanism for multiple domain types
• Makes it easier to swap serialization formats (JSON, Protobuf, etc.)
• Improves testability and maintainability of serialization logic