fix: added a new pkce string generator and use that in oauth handshake

[battkajs]

Closes #5322

📑 Description

Added a helper function to generate a random string of 128 characters compliant with RFC7636: https://datatracker.ietf.org/doc/html/rfc7636

✅ Checks

• [ x] My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.61%. Comparing base (57ca737) to head (197e89d).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5323       +/-   ##
===========================================
- Coverage   46.64%   30.61%   -16.04%     
===========================================
  Files         175      101       -74     
  Lines       18263    11638     -6625     
===========================================
- Hits         8519     3563     -4956     
+ Misses       9744     8075     -1669     

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[shahargl]

lgtm

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Project	Deployment	Preview	Comments	Updated (UTC)
keep	Ready	Preview	Comment	Sep 20, 2025 3:42am

[shahargl]

@battkajs can you sign CLA so I can merge?

[battkajs]

@shahargl I had forgot to add my email to the git config. Could this be bypassed or should I re-do the PR?

EDIT: I did a git amend with my email present, I have successfully accepted the terms but the bot workflow dont pick it up.

[github-actions]

🌟 Well done @battkajs! Two PRs merged already! 🎉🥳

With your second PR, you're on a roll, and your contributions are already making a difference. 🌟
Looking forward to seeing even more contributions from you. See you in Slack https://slack.keephq.dev 🚀