Fix dynamic client registration returning hashed secret when hash_application_secrets is enabled

[Copilot]

When Doorkeeper's hash_application_secrets is enabled, the dynamic client registration endpoint was returning the BCrypt hash instead of the plaintext secret, making the registered client unusable.

Changes

• Controller: Use plaintext_secret || secret in registration_response — plaintext_secret is populated by Doorkeeper on the in-memory object immediately after create!, before the secret is hashed and persisted. Falls back to secret when hashing is disabled.
• Spec: Updated assertion to match.

# Before
client_secret: doorkeeper_application.secret,

# After
client_secret: doorkeeper_application.plaintext_secret || doorkeeper_application.secret,

This mirrors the fix Doorkeeper applied to its own UI flow in #1246.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Dynamic client registration broken with hash_application_secrets</issue_title>
<issue_description>Hey! I ran into an issue where dynamic client registration returns the hashed secret instead of the plaintext when hash_application_secrets is enabled in Doorkeeper.

doorkeeper-openid_connect/app/controllers/doorkeeper/openid_connect/dynamic_client_registration_controller.rb

Line 31 in 9927111

client_secret: doorkeeper_application.secret,

This should probably be:

client_secret: doorkeeper_application.plaintext_secret || doorkeeper_application.secret,

Same issue was fixed for the UI flow back in 2019: doorkeeper-gem/doorkeeper#1246

Happy to submit a PR if this looks right to you!
</issue_description>

<agent_instructions>Can you analyze the issue and prepare a fix? </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@nbulaj Hello @lionelvoser . True! PR is very welcome, thanks! 🙇

• Fixes Dynamic client registration broken with hash_application_secrets #231

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[nbulaj]

LGTM