Invoke conversion methods that take an IFormatProvider or optional parameters

[matt-edmondson]

Fixes #95

Problem

FindStringConversionMethod accepted any FromString/Parse/Create/Convert method whose first parameter is a string. Read and ReadAsPropertyName then always called it with exactly one argument. So CanConvert claimed types it could not deserialize, and deserializing them threw TargetParameterCountException. Two common shapes hit this:

• types with only the IParsable<T> shape, Parse(string, IFormatProvider?)
• factories with an optional second parameter

Change

• Overload choice: within each supported method name, a single-string overload is now preferred. Before, whichever overload GetMethods() returned first was used, and that order is unspecified.
• Which methods are accepted (IsUsableConversionMethod): a method qualifies only if every parameter after the string is an IFormatProvider or optional. For any other shape, such as Create(string, int) with a required int, CanConvert now returns false.
• How the method is called (BuildArguments): an IFormatProvider parameter gets CultureInfo.InvariantCulture, and an optional parameter gets Type.Missing, which makes the runtime use its declared default. Read and ReadAsPropertyName both use this.
• New ExtraParameterConversionTests:
  • an IParsable-shaped round trip, as a value and as a dictionary key
  • a Create(string, int weight = 7) round trip that gets the default weight
  • rejection of a required extra parameter
  • the single-string overload winning over the IFormatProvider overload

Verification

• With the library change reverted, all 5 new tests fail, four of them with TargetParameterCountException. With it in place, they pass. The overload-preference test also failed on main, which confirms the GetMethods() ordering problem is real.
• Full suite: 101/101 passed on net10.0.

Not included

The triage suggested culture-invariant IFormattable serialization on the write side as a follow-up. It changes the output format, so I left it out of this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_011wnB34hMv7jTvqsaysM9MS

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
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100.0% Coverage on New Code
0.0% Duplication on New Code

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