[v3-1-test] Fix connection test API to restore masked password/extra from existin…

[pierrejeambrun]

…g connections (#59643)

• Fix connection test API to restore masked password/extra from existing connections

• Use get_connection_from_secrets in test endpoint

• Remove unused session parameter

• Add use_existing_credentials flag to test connections

• fix api-gen description

• Standardize docstring format

• Add use_existing_credentials flag to test connections

• Refactor connection test logic using update_orm_from_pydantic

• Remove useExistingCredentials flag from TestConnectionButton mutation

• Add proper assertions for test connection merge tests

• Small test adjustment

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(cherry picked from commit efb27dc)

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