[codex] Fix decimal correctness and maintenance issues

[sdcoffey]

Summary

• Preserve precision when constructing, formatting, and combining decimal values.
• Normalize zero-value and NaN handling across arithmetic, JSON, SQL, and comparisons.
• Handle invalid math cases such as negative square roots and negative powers.
• Reduce internal dependence on mutable exported sentinels.
• Update github.com/stretchr/testify to v1.11.1.
• Pin Makefile tool installs, stop make test from rewriting files, and harden release tag detection.
• Clarify README semantics for the math/big.Float-backed implementation.

Commit Structure

Each review finding/fix area is split into its own commit for easier review.

Validation

• mise exec go@1.26.3 -- go test ./...
• bash -n scripts/release.sh
• Confirmed GitHub Actions Tests matrix passes on Go 1.21, 1.22, 1.23, 1.24, 1.25, and 1.26.

[chatgpt-codex-connector]

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