fix(data-format): preserve values when expanding scientific notation

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Changes

• Expand floats to fixed-point decimal strings before trimming fractional trailing zeros, preserving their numeric values in query results, live chart data and Excel exports.
• Add 13 regression cases covering positive and negative exponents, signed values, zero, conversion thresholds and nested rows.

Reason

Fixes #1387.

The previous formatter could retain scientific notation and strip zeros from its exponent: 1.23e20 became 1.23E+2, and 1.23e-10 became 1.23E-1. Formatting with f removes the exponent before trimming zeros from the fractional part.

Validation

• Python 3.11: the new regression file failed 8 cases before the fix; all 13 cases pass after it.
• Full test suite (backend/tests and tests): 217 passed, 3 skipped, 1 pre-existing failure in ExecutionErrorDetailsTestCase.test_ai_log_skips_normal_content_after_an_error. The same failure reproduces on unmodified base 2ce3fc71 (204 passed, 3 skipped, 1 failed). MiniMax API tests were skipped without credentials.
• XLSX write/read check using the production formatting helpers, pandas, xlsxwriter and openpyxl: all 7 sample values preserved; converted values remain text cells.
• Ruff check and format check pass for the new test file; git diff --check passes. The production file retains the same 5 pre-existing Ruff findings as the base.
• Full application endpoints were not exercised.