test: unit and property coverage for strategies

[Ishant5436]

adds unit and property-based test coverage for the constant_spread and adaptive_spread quoting strategies. the property tests uncovered a type bug in adaptive_spread that is fixed in this pr.

fixes #29

[abhicris]

🤖 Audit verdict: safe

Standard test coverage PR with legitimate property-based testing, one helpful type-casting fix for JSON serialization, no malicious code or supply-chain risks.

Audited by the kcolbchain PR pipeline. See pipeline docs.

[abhicris]

Welcome to kcolbchain, @Ishant5436 — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
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