Fix negative counter handling

[neelp03]

Summary

• prevent negative values in MemoryStore.Increment
• test negative increments for new keys

Testing

• go test ./... (fails: Get "https://proxy.golang.org/...": Forbidden)

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https://chatgpt.com/codex/tasks/task_e_6870cf2099d883298a522ff278a3a978

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
store/memory.go	66.66%	2 Missing and 1 partial ⚠️

Flag	Coverage Δ
unittests	73.74% <66.66%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
store/memory.go	96.36% <66.66%> (-2.67%)	⬇️

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