fix(datasource): require POST to close connections

[HandSonic]

Summary

• Change /api/connection/close from GET query parameters to a validated POST JSON body because it removes an active connection.
• Update the sole frontend caller contract for both Web and desktop IPC transports.
• Add focused frontend and backend contract coverage.

Verification

• DbDataSourceControllerTest: 1 test passed through the Maven reactor.
• yarn test:connection-close-request passed.
• Targeted ESLint and git diff --check passed.
• Independent adversarial review: PASS, 0 blockers.
• Fork CI: backend, frontend, Java/JavaScript CodeQL, repository/docs, licenses, and SBOM passed.

Fixes #2678

[openai0229]

The GET-to-POST contract change itself matches #2678: the frontend sends POST /api/connection/close with { id }, and the controller delegates to removeConnection(id). There is no logic blocker in that focused change.

The branch is currently CONFLICTING with main, so it cannot merge as-is. Please rebase and preserve the newer controller/client changes, then rerun the focused frontend request-contract and controller tests. The separate legacy GET /datasource/close endpoint remains a pre-existing no-op and should be handled independently rather than folded into the conflict resolution.