Add MA0193 Round mode analyzer and code fixes

[meziantou]

Why

Round without an explicit MidpointRounding mode can produce surprising results because it defaults to ToEven. This PR adds a dedicated rule so call sites must choose the rounding behavior intentionally.

What changed

• Added MA0193 analyzer to report Round overloads that omit MidpointRounding for:
  • Math.Round(...)
  • MathF.Round(...)
  • decimal.Round(...)
  • IFloatingPoint<TSelf>.Round(...) and implementations
• Added code fixes for MA0193 with one suggestion per available System.MidpointRounding member (for example ToEven, AwayFromZero, ToZero, ToNegativeInfinity, ToPositiveInfinity).
• Enabled MA0193 by default as a suggestion.
• Added/updated docs for MA0193 and regenerated generated docs/config sections.
• Added tests covering analyzer behavior and per-enum-member code fix choices.

Notes for reviewers

• The fixer inserts the enum argument at the overload position computed via OverloadFinder, so it works for both framework methods and IFloatingPoint<TSelf> implementations.
• The MA0193 tests verify each code fix action index maps to a distinct MidpointRounding value.