Change TriggerState finished bitset coder to a SentinelBitSetCoder

[arunpandianp]

SentinelBitSetCoder is same as BitSetCoder except that it encodes empty bitset as a single element 0 byte array.

This allows checking if the finished bitset is empty or missing #37574.

SentinelBitSetCoder and BitSetCoder are state compatible. Both coders can decode encoded bytes from the other coder successfully.

[gemini-code-assist]

Summary of Changes

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This pull request introduces a specialized coder, SentinelBitSetCoder, to handle BitSet serialization in scenarios where distinguishing between an empty bitset and a missing one is required. By ensuring backward compatibility with the existing BitSetCoder, this change allows for safer state management within the trigger state machine runner without breaking existing data pipelines.

Highlights

• New Coder Implementation: Introduced SentinelBitSetCoder, which encodes empty BitSet objects as a single-element 0 byte array to distinguish between empty and missing states.
• Compatibility: Ensured that SentinelBitSetCoder remains state-compatible with the existing BitSetCoder, allowing for seamless decoding of data encoded by either implementation.
• Refactoring: Updated TriggerStateMachineRunner to utilize the new SentinelBitSetCoder for the FINISHED_BITS_TAG.

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[arunpandianp]

R: @scwhittle

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