refactor: add ErrMessageTooBig sentinel error for limited reads

[DanielleMaywood]

This change allows detecting when a message exceeds the maximum allowed size
during reads via errors.Is(err, ErrMessageTooBig). Previously, this condition
required string matching against the error message. For backwards compatibility,
the old message is preserved in the error chain.

[johnstcn]

Deferring approval to @mafredri

[mafredri]

Considering this code-path closes the connection, what use-case are we addressing by detecting the specific error?

[DanielleMaywood]

Considering this code-path closes the connection, what use-case are we addressing by detecting the specific error?

The exact use-case is for better logging here:
https://github.com/coder/coder/blob/205eb29e60cc3b3519b4ff0247d191b0c87e4d75/codersdk/wsjson/decoder.go#L39-L41

It was previously decided to log at a debug log level. Assuming we want to keep that original behavior, but raise it for this specific error, we want to be able to match against it properly (without using string.Contains on the error).