chore: signmode textual audit fixes

[facundomedica]

Description

Waiting for #15550 to get merged first, although those changes have been pulled here already to make it r4r faster.

All these changes are small things related to code linting, style, docs, etc. No big issues found.

• Use messageValueRenderer's header() instead of fmt.Sprintf("%s object", internalMsg.ProtoReflect().Descriptor().Name()). It's the same thing given that the descriptor is stored in the value renderer and then header() returns fmt.Sprintf("%s object", mr.msgDesc.Name()).
• In GetSignBytes use NewTxValueRenderer instead of getting it through GetMessageValueRenderer .
• Remove unnecessary interface checks
• Fix names, use existing variables instead of raw structs for nilValue, other misc fixes
• Added a couple tests.

Yet to be solved:

• Check if MessageValueRenderer formatRepeated could conform to the RepeatedValueRenderer interface

Closes: #XXXX

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