The prettify/merge/copy/save buttons sat at the far right of the tab
strip, over the response pane — far from the query you're editing.
Split the sessions area into an editor column and a response column
divided by the drag bar, and move the tab strip into the editor column
so the actions land at the editor's right edge and follow the divider
as it's resized.

Reconcile the two-column sessions restructure with the flattened,
flush editor frame that landed on graphiql-6 (#4397, #4405):

- Drop the 8px inset on `.graphiql-session-row` so the panes sit flush
  to the frame edges, matching the inset-padding removal (#4405).
- Move the editor/response divider `border-right` from `.graphiql-editors`
  onto `.graphiql-editor-column` so it runs the full column height
  (header + editor) rather than only the editor body.
- Keep `min-height: 0` on `#graphiql-session`.

The editorFlex resize ref moved from .graphiql-editors onto the new
.graphiql-editor-column wrapper, so the panel-resizing test now reads
flexGrow from the column.

Drop the empty spacer above the response pane. The response header
(the JSON/Tree/Table view picker) now sits at the top of the column,
matched to the session header's height so it lines up with the tabs
and action buttons across the split.

Moving the editor/response resize onto the column wrappers left
`.graphiql-response` without a definite height, so it shrank to just
its header. The Tree/Table views still showed (intrinsic content
height), but the Monaco JSON result window is `flex: 1` of that
collapsed box and rendered at zero height — a blank JSON view. Give
`.graphiql-response` `flex: 1; min-height: 0` so it fills the column,
mirroring the editor side.

… header

Give the session header (tabs + actions) the elevated surface and bottom
divider the response header already uses, so both read as matching toolbar
strips across the split. Drop the response header's left padding so its
status/view-picker sits flush with the pane edge, like the tabs on the
editor side.

Two header-strip fixes:
- The tab strip carried its own shorter height + bottom border, which
  double-lined against the header divider. Stretch it to the full header
  height and drop its border so there's a single divider.
- The editor/response drag bar took 8px of layout width, pushing the
  response header off the divider. Make it straddle the divider (negative
  margins, unchanged width) so the response sits flush; the resize math
  reads the bar's clientWidth, which is untouched.

The editor/response and plugin/sessions resizers used a dedicated 8px-wide
drag bar (or, latterly, a straddling negative-margin hack), so they left a
gap or a bar floating over the divider and were inconsistent with each other.

Restyle `.graphiql-horizontal-drag-bar` as a sash: zero layout width so the
panes sit flush, an invisible 8px hit area (`::before`) straddling the seam,
and a 1px divider line (`::after`) that thickens on hover. Width 0 keeps the
resize math (which subtracts the bar's clientWidth) pixel-exact. The divider
is now drawn by the sash, so the editor-column `border-right` is dropped.

The col-resize cursor was only on the sash's ::before (unreliable on
pseudo-elements); move it onto the drag-bar element so it shows over the
hit area. Restore the response header's default horizontal padding so the
status dot no longer runs into the divider now that the pane is flush.

Resolve the query-editor conflict: keep PR 84's removal of the legacy
`.graphiql-toolbar` + `ExecuteButton` (Run lives in the top bar now), and
take graphiql-6's `QueryEditor` call, which drops the removed
`onClickReference` prop.