[core][doc] Add raises to docstrings and fix related issues

[forfudan]

This PR focuses on documentation correctness and clarity across Decimo’s Mojo codebase, primarily by expanding docstrings with explicit Raises: sections and converting some file header comments into proper module docstrings.

Changes:

• Add/standardize Raises: sections across many public APIs (BigDecimal/BigInt/BigUInt/Decimal128/TOML utilities, CLI engine, test helpers).
• Improve module-level documentation by replacing banner-style comments with descriptive module docstrings and adding missing package/module docstrings.
• Minor benchmark helper adjustment: _pad() now uses String.byte_length() when computing padding.

[Copilot]

Pull request overview

This PR focuses on documentation correctness and clarity across Decimo’s Mojo codebase, primarily by expanding docstrings with explicit Raises: sections and converting some file header comments into proper module docstrings.

Changes:

• Add/standardize Raises: sections across many public APIs (BigDecimal/BigInt/BigUInt/Decimal128/TOML utilities, CLI engine, test helpers).
• Improve module-level documentation by replacing banner-style comments with descriptive module docstrings and adding missing package/module docstrings.
• Minor benchmark helper adjustment: _pad() now uses String.byte_length() when computing padding.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
tests/bigdecimal/test_bigdecimal_methods.mojo	Docstring wording/punctuation tweaks for the test module header.
tests/bigdecimal/test_bigdecimal_arithmetics.mojo	Docstring list formatting tweaks.
src/decimo/toml/parser.mojo	Adds Raises: sections to TOML public APIs and parse helpers.
src/decimo/tests.mojo	Adds Raises: sections to shared test/benchmark helpers.
src/decimo/rational/rational.mojo	Adds Raises: sections for Rational arithmetic operators.
src/decimo/rational/comparison.mojo	New module docstring for Rational comparison operations.
src/decimo/rational/arithmetics.mojo	New module docstring for Rational arithmetic operations.
src/decimo/decimal128/utility.mojo	Replaces header comments with a module docstring.
src/decimo/decimal128/decimal128.mojo	Adds Raises: sections for Decimal128 operations.
src/decimo/decimal128/init.mojo	Adds package docstring for decimal128 subpackage.
src/decimo/biguint/biguint.mojo	Adds Raises: sections for multiple BigUInt methods.
src/decimo/biguint/arithmetics.mojo	Adds Raises: sections to division helpers.
src/decimo/bigint10/bigint10.mojo	Adds Raises: sections and expands docstring(s).
src/decimo/bigint/number_theory.mojo	Adds Raises: sections for number theory helpers.
src/decimo/bigint/bigint.mojo	Adds Raises: sections for conversion and modular ops.
src/decimo/bigfloat/bigfloat.mojo	Adds Raises: sections for conversions and power forwarding.
src/decimo/bigdecimal/trigonometric.mojo	Adds module docstring and Raises: sections for trig functions.
src/decimo/bigdecimal/rounding.mojo	Adds Raises: sections to rounding helpers.
src/decimo/bigdecimal/exponential.mojo	Adds Raises: sections to cache and exponential helpers.
src/decimo/bigdecimal/constants.mojo	Adds Raises: sections for π computation helpers.
src/decimo/bigdecimal/bigdecimal.mojo	Adds Raises: sections across many BigDecimal APIs.
src/decimo/bigdecimal/arithmetics.mojo	Adds Raises: sections to arithmetic helpers.
src/cli/calculator/engine.mojo	Documents propagated errors for evaluation helpers.
benches/decimal128/mojo/bigdec_ref.mojo	_pad() now uses byte_length() when computing padding.

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