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 56.66667% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.97%. Comparing base (dbedcf2) to head (97cd530).
⚠️ Report is 2 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   64.43%   64.97%   +0.54%     
==========================================
  Files         214      214              
  Lines       18108    18118      +10     
==========================================
+ Hits        11667    11773     +106     
+ Misses       5348     5250      -98     
- Partials     1093     1095       +2     

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[zimeg]

@srtaalej LGTM! This is amazing 🧪 ✨

Thank you sincere for bringing these patterns to the CLI - it's a huge improvement on our previous testing of prompts: IIRC we just covered the flag cases... 🏁

[zimeg]

⭐ praise: Thanks so much for keeping the formats to our outputs together here!