[sidracha]

tests: add reconciliation unit tests and concurrent op head integration tests

Add unit tests in server/src/reconcile_op_heads.rs that directly exercise reconcile_repo_op_heads(). One test verifies the single-head fast path, where reconciliation is a no-op and the existing head is returned without invoking jj-lib. Another test sets up two divergent operation heads in the store and asserts that the server's reconciliation produces exactly the same merged operation as calling jj-lib's resolution directly, confirming parity with jj-lib. A failure-path test registers a divergent head whose underlying operation object is missing from the store, and verifies that reconciliation retries MAX_RETRIES times and then surfaces an error to the caller rather than silently succeeding.

Add integration tests in cli/tests/test_concurrent_op_heads.rs that run against a live SQLite-backed server. The first test has two clients race to advance the same op head, and verifies that the stale write is rejected with a CAS conflict. A stress test fires many concurrent UpdateOpHeads RPCs at once and verifies that no writes are lost. Another test confirms that ReconcileOpHeads is a no-op when only a single op head exists for the repo. The final test is end-to-end: it creates divergent op heads in a real workspace, runs a jj CLI command, and confirms that the client automatically triggers server-side reconciliation, collapsing the divergent heads into a single merged operation.