feat(bigtable): route single-entry MutateRows through a point-write c…

[mutianf]

…allable

Add MaybePointWriteCallable, mirroring MaybePointReadCallable: a BulkMutation with exactly one entry is converted to a RowMutation and dispatched through a point-write callable so it can benefit from the session-shim diversion, while multi-entry bulk mutations continue through the classic MutateRows path.

The point-write callable falls back to the MutateRow RPC when the session diversion does not apply, but carries the caller's bulkMutateRowsSettings retry settings/codes so a single-entry bulk write retries the same way it would have as a MutateRows call.

[gemini-code-assist]

Code Review

This pull request introduces MaybePointWriteCallable to route single-entry BulkMutations through a unary point-write callable (MutateRow RPC) instead of the classic bulk mutation path, allowing them to benefit from session-shim diversion. It also updates EnhancedBigtableStub to integrate this new callable and adds corresponding unit tests. The review feedback suggests adding a test case to verify that a BulkMutation with zero entries is correctly routed to the classic bulk mutation path.

[gemini-code-assist]

Add a test case to verify that a BulkMutation with zero entries is correctly routed to the classic bulk mutation path instead of the point-write path.

  @Test
  public void multipleEntries_fallsThroughToClassic() {
    BulkMutation request =
        BulkMutation.create(TABLE_ID)
            .add("row-a", Mutation.create().deleteRow())
            .add("row-b", Mutation.create().deleteRow());

    callable.futureCall(request, null);

    assertThat(pointWriter.request).isNull();
    assertThat(classic.request).isEqualTo(request);
  }

  @Test
  public void zeroEntries_fallsThroughToClassic() {
    BulkMutation request = BulkMutation.create(TABLE_ID);

    callable.futureCall(request, null);

    assertThat(pointWriter.request).isNull();
    assertThat(classic.request).isEqualTo(request);
  }