tests(charm): adds tests to charm experiment

[srtaalej]

Changelog

Added unit tests for charm prompt implementations and Slack brand theme 🦋

Summary

This PR adds unit tests for all 5 charm/huh prompt types (input, confirm, select, password, multi-select) and the Slack brand theme. It refactors charm prompt functions to extract buildXxxForm builders, separating form construction from execution to enable programmatic testing via huh's Update/View API

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 86.45833% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.76%. Comparing base (1059ec2) to head (94b0d7d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/iostreams/charm.go	56.66%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #355      +/-   ##
==========================================
+ Coverage   64.48%   64.76%   +0.27%     
==========================================
  Files         213      214       +1     
  Lines       17879    17955      +76     
==========================================
+ Hits        11530    11629      +99     
+ Misses       5276     5248      -28     
- Partials     1073     1078       +5     

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