feat: add linear service

[eryue0220]

This pr is to add new linear service and e2e node.

[mbakgun]

Hi @eryue0220 🥇 Thanks for the work on this PR. The Linear integration is useful and the API key node path looks close to mergeable. I’d like a few OAuth fixes before we land it. 🚀

Could you please address these? 👑

• Fix the OAuth token refresh request. The refresh call reuses the GraphQL client, which carries a default Content-Type: application/json, so the body goes out form-encoded while the header says JSON. Linear’s token endpoint expects application/x-www-form-urlencoded, so refresh will fail once a token expires. I reproduced this locally. The initial callback is fine since it uses a fresh client, so only the refresh path needs an explicit form content type or a separate token client.
• Confirm Linear’s scope delimiter. The code uses space-separated scopes, while Linear’s docs appear to use comma-separated scopes such as read,write. Please verify this and update both the scope string and the test if needed.
• Add a refresh-path test. That path has no coverage right now. Please verify the request headers and body, and that refreshed tokens are persisted back to the credential.
• Close the LinearService client in the credential connection test path after test_connection(), matching the executor’s finally: service.close() pattern.
• Make the docs consistent about Linear supporting both API key and OAuth, or split OAuth into a follow-up if we only want to ship API key support here.

Overall I’m happy with the direction 🙏 OAuth should either be corrected here or moved to a follow-up PR so the API key node can land cleanly. ⭐️