[v2.0] Refactor Jira Get*Settings to use typed struct

Goal

Transition the Jira integration settings to a dedicated struct for v2.0.

Instructions

  1. Investigation: Run an API GET request against a configured Jira integration. Document the exact JSON keys found in the properties block in a comment below.
  2. Struct Definition: Create a JiraIntegration struct that embeds Integration and a JiraIntegrationProperties struct using the verified keys.
  3. Refactor: Update the GetProjectJiraSettings and GetGroupJiraSettings return types.
  4. Validation: Regenerate mocks and add unit tests covering the new struct.

Parent issue: #2198

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