Do not offer the MA0044 code fix when removing ToString would drop argument side effects

[meziantou]

Problem

MA0044 reports any ToString call on System.String, including ToString(IFormatProvider). The provider is ignored by that overload, but C# still evaluates the argument. The fixer replaced the whole invocation with its receiver, so side effects of the argument were silently removed:

_ = "text".ToString(Provider()); // Provider() is called
// after the fix
_ = "text";                      // Provider() is no longer called

Changes

• RemoveUselessToStringFixer now only registers the fix when every argument can be discarded without observable effect: constants and literals, locals, parameters, this, static fields or fields of this, and static CultureInfo properties (so str.ToString(CultureInfo.InvariantCulture) is still fixed). Implicit conversions are looked through unless they call a user-defined operator.
• The member-access check is moved from the fix action to the registration, per the repository guidance. As a result the fix is no longer offered for value?.ToString(), where it previously returned the document unchanged.
• The analyzer is unchanged: the call is still useless, so the diagnostic is still reported.
• docs/Rules/MA0044.md documents when the fix is not offered.

Tests

Added to RemoveUselessToStringAnalyzerTests:

• no fix for a side-effecting provider call, a property provider, and ?.ToString() (these three fail against the previous fixer)
• fix applied for CultureInfo.InvariantCulture, null, and a parameter

The test class passes (8/8) on Roslyn 4.8, 4.14, 5.0, 5.6 and 5.9. The full suite was not run locally.

Known gaps (not addressed here)

• The fix still removes the call when the receiver may be null (s.ToString() throws a NullReferenceException, s does not).
• Reading a static field may run a static constructor.