Honor exception: false for complex conversion

[OskarEichler]

Summary:

• return nil for Complex values with a nonzero imaginary part when exception: false
• preserve ArgumentError for the default and exception: true paths

Why:
Kernel.BigDecimal accepts exception: false as a non-raising conversion mode, but this Complex branch contains a TODO and always raises. That differs from the documented option and the method’s other invalid-input paths.

Verification:

• baseline reproduction raises ArgumentError for BigDecimal(Complex(1, 1), exception: false)
• fixed reproduction returns nil; exception: true still raises; zero-imaginary conversion is unchanged
• full current suite: 266 tests, 13,616 assertions, 0 failures/errors/omissions
• RBS suite: 146 tests, 1,805 assertions, 0 failures/errors

Compatibility:
Only the explicitly non-raising invalid-conversion path changes.