Errors due to placeholders for unlinked libraries in bytecode

When a Solidity library has functions declared external or public (rather than only internal functions), it must be linked by providing the compiler with the address where the library is deployed (see Library Linking in the Solidity documentation). Until it is linked, the bytecode of a contract using the library will contain placeholders of the form __$53aea86b7d70b31448b230b20ae141a537$__ instead of the library address. Trying to execute this bytecode using KEVM leads to two issues:

1. The definedness constraint added by foundry-prove (here) evaluates to #Bottom, causing the entire initial configuration to simplify to #Bottom. I believe this happens because #parseByteStack is not defined when the string includes non-numerical characters.
2. foundry-show can fail because it tries to read a numerical byte and gets something that contains a special character instead, like __ or $5 (see runtimeverification/evm-semantics#1895).

To avoid these issues, which are non-trivial for a user to debug, it would be better if we could detect when the bytecode contains placeholders for unlinked libraries and give the user an informative error message.

Start with the definedness constraint in kevm-pyk/src/kevm_pyk/foundry.py around line 863 and inspect how foundry-prove processes bytecode containing Solidity library placeholders. Also trace the foundry-show failure described in runtimeverification/evm-semantics#1895. Done means unlinked-library placeholders are detected and users receive an informative error instead of the current failures.