core: a refused argument form (undefined for null, a bigint for a display nat) shares a query key with the valid call, so it can be served cached data

[b3hr4d]

Found while fixing #761/#762 (PR #764). Everything here happens on main.

1. A refused argument can share a key with a valid call

• undefined where Candid null is required gets the same query key as null, in both Reactor and DisplayReactor. The codec refuses undefined there, so a direct call fails.
• A bigint for a display nat gets the key of its decimal text (core: query keys map NaN/Infinity/-0 to one key, and give equal records in different field order different keys #515 writes bigints as decimal strings). The DisplayReactor codec refuses the bigint.

When the valid call's result is cached, the refused form is answered from the cache through fetchQuery or a query hook, instead of failing. So an app with a bad argument can look like it works, until the cache is empty and the call throws.

Suggested fix: key a value only by the form the codec accepts at that position, and give refused values a key that can't match an accepted one. #757 and #764 already read argument types in the key builder, so this is local.

2. Some forms that send the same bytes still get separate keys

Extra calls, not wrong data:

• extra record fields
• a number vs numeric text for integers of 32 bits or less, and for floats
• a Principal vs its text
• a typed array vs a plain array for non-blob vectors
• different values of reserved

Acceptance

• A refused form never shares a key with an accepted one. The test fails on main.
• Item 2 is fixed where it's cheap, or documented.