CDAP-21256 : Support preserving client credentials when deleting an OAuth provider

[sahusanket]

Testing :

• Create provider with Cred :

curl -X PUT "${BASE_URL}/${PROVIDER}?reuse_client_credentials=false" \
  -H "Content-Type: application/json" \
  -d '{
    "loginURL": "https://login.salesforce.com/services/oauth2/authorize",
    "tokenRefreshURL": "https://login.salesforce.com/services/oauth2/token",
    "clientId": "3MVG9lKcPoaZ_ORIGINAL_ID",
    "clientSecret": "super_secret_password_v1",
    "strategy": "FORM_BODY",
    "userAgent": "CDAP-Pipeline/1.0"
  }'

• Delete

curl -X DELETE "${BASE_URL}/${PROVIDER}?preserve_client_credentials=true"

• Create provider with reuse_client_credentials=true

curl -X PUT "${BASE_URL}/${PROVIDER}?reuse_client_credentials=false" \
  -H "Content-Type: application/json" \
  -d '{
    "loginURL": "https://login.salesforce.com/services/oauth2/authorize",
    "tokenRefreshURL": "https://login.salesforce.com/services/oauth2/token",
    "strategy": "FORM_BODY",
    "userAgent": "CDAP-Pipeline/1.0"
  }'

SUCCESS

If the secrets were not present in backend, it will throw an error.

[gemini-code-assist]

Code Review

This pull request introduces a preserve_client_credentials option when deleting an OAuth provider, allowing users to preserve existing secrets in the secure store. This is implemented by adding a preserveClientCredentials parameter to OAuthStore.deleteProvider and exposing it as a query parameter in the OAuthHandler DELETE endpoint, along with corresponding unit tests. The review feedback suggests changing the query parameter type in OAuthHandler from boxed Boolean to primitive boolean to avoid a potential NullPointerException during auto-unboxing, which would otherwise result in an incorrect 'Invalid provider' error response.