fix: reject malformed amount strings in validateStringAmount

[Snowmanzx]

validateStringAmount used parseFloat to check the amount. parseFloat reads a leading numeric prefix and ignores the rest, so "1abc" becomes 1, "1.2.3" becomes 1.2 and "1e6" becomes 1000000. Those pass validation and only fail later in parseUnits, which expects a plain decimal string. A commenter on the issue also flagged the "1e6" scientific notation case.

Replaced the parseFloat check with a strict decimal regex. The rest of the behavior is unchanged on purpose: negative and zero amounts still throw "must be greater than 0", non string input still throws "must be a string", and the max decimal places check is untouched. All existing tests still pass.

Added a test that runs the malformed inputs through the validator and checks they are rejected.

Closes #313

[cb-heimdall]

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