Implement missing merge/merge! functions for AlgR and AlgL unweighted sampling algorithms

[Copilot]

This PR implements the missing merge and merge! functions for the unweighted reservoir sampling algorithms AlgR and AlgL that were previously throwing "To Be Implemented" errors.

Problem

The MultiAlgRSampler (AlgR) and MultiAlgLSampler (AlgL) classes had placeholder implementations for their merge functions:

function Base.merge(ss::MultiAlgRSampler...)
    error("To Be Implemented")
end
function Base.merge(ss::MultiAlgLSampler...)
    error("To Be Implemented")
end
function Base.merge!(ss::MultiAlgRSampler...)
    error("To Be Implemented")
end
function Base.merge!(ss::MultiAlgLSampler...)
    error("To Be Implemented")
end

This prevented users from merging reservoir samplers using these algorithms, while the functionality was available for other algorithms like AlgRSWRSKIP.

Solution

1. Added probability calculation functions

Implemented get_ps functions for both algorithms in SamplingReduction.jl:

• Uses seen_k (number of items processed) as the basis for merge probabilities
• Includes handling for empty reservoirs by using uniform probabilities when sum of seen_k is 0

2. Implemented merge functions

Added Base.merge functions following the same pattern as existing implementations:

• For AlgR: aggregates seen_k and uses reduce_samples for value combination
• For AlgL: additionally aggregates state and skip_k fields specific to Algorithm L

3. Implemented merge! functions

Added Base.merge! functions with proper type constraints:

• Uses {<:Nothing} constraint for unordered reservoirs (matching existing pattern)
• Mutates the first sampler in-place and returns it

4. Added comprehensive tests

Extended merge_tests.jl with tests for both algorithms covering:

• Empty reservoir merging
• In-place mutation behavior verification
• Integration with existing test infrastructure

Testing

All tests pass (370/370), including the new tests for AlgR and AlgL merge functionality. The implementation correctly handles edge cases like empty reservoirs and maintains the same behavior patterns as existing merge implementations.

# Now works without errors
s1 = ReservoirSampler{Int}(rng, 2, AlgR())
s2 = ReservoirSampler{Int}(rng, 2, AlgR())
merged = merge(s1, s2)  # ✅ Works
merge!(s1, s2)          # ✅ Works

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