Correct nullable BigDecimal conversion signature

[OskarEichler]

Summary:

• distinguish the default/exception: true overload from non-literal boolean calls
• return BigDecimal? whenever exception may be false
• add a conservative untyped fallback for the dynamic Kernel conversion API

Why:
Kernel.BigDecimal returns nil for failed conversions when exception: false, but the current RBS always declares BigDecimal. The overload shape follows the established Float, Integer and Complex signatures in core RBS.

Verification:

• full current suite: 266 tests, 13,616 assertions, 0 failures/errors/omissions
• RBS suite: 146 tests, 1,805 assertions, 0 failures/errors
• conversion model covers literal true, literal false, a boolean variable and dynamic inputs

Compatibility:
This changes type information only. Existing calls remain accepted while non-raising conversions receive their actual nullable return type.