Fix exponential notation for numbers not ending with zeros (issue #236)

[devin-ai-integration]

Summary

Fixes #236 where numbers like 998244353 were incorrectly being converted to exponential notation (e.g., "9.98244353 × 10^8").

The fix adds an additional condition to the exponential notation conversion: numbers are only converted if they end with enough trailing zeros based on the exponential_threshold setting. For the default threshold of 1000000, numbers must end with at least 6 zeros to be converted.

Changes:

• Modified to_string() in variables_converter.py to check that numbers end with threshold_k zeros (where 10^threshold_k >= exponential_threshold)
• Added validation in StatementConfig to reject exponential_threshold < 1
• Added tests for the fix and validation

Review & Testing Checklist for Human

• Verify edge case with exponential_threshold = 1: When threshold is 1, threshold_k = 0, so endswith("") is always True. This means all numbers >= 1 would be converted to exponential notation. Confirm this is acceptable behavior.
• Test with real problem.toml files: Run ss-manager on existing problem sets that use constraints like MOD = 998244353 to verify they now display correctly as "998,244,353" instead of exponential notation.
• Verify the logic matches issue discussion: The approach was agreed upon in issue 998244353 が 9.98244353 × 10^8 になる #236 comments - confirm the implementation matches the expected behavior.

Recommended test plan:

1. Create a problem.toml with MOD998 = 998_244_353 and LARGE_CLEAN = 10_000_000
2. Run ss-manager run and verify MOD998 displays as 998{,}244{,}353 (not exponential) and LARGE_CLEAN displays as 10^{7} (exponential)

Notes

• Link to Devin run: https://app.devin.ai/sessions/a7747b041c7b414ba00f2e60a20648a7
• Requested by: @tsutaj

[devin-ai-integration]

Original prompt from Yuya

Received message in Slack channel #tech:

@Devin statements-manager の issue 236 をやってください
アプローチが何通りか考えられるので、まずどのような方針でやろうとしてるか教えてください
Thread URL: https://tsutaj-playground.slack.com/archives/C02EUD46XS9/p1768009432958019

[devin-ai-integration]

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