… (integrated-charts dark mode) (#14917)

* AG-18256 - Apply the example colour scheme before the example runs

Integrated-charts docs examples rendered a light chart inside a dark grid, or
flickered light-then-dark. The example page's `data-ag-theme-mode` was only ever
written by the parent frame's iframe `load` handler, which runs after the
example's deferred entry module has already created its grid and chart.

- Carry the resolved colour scheme on the example iframe `src` as
  `?agThemeMode=`, composed by a pure `withThemeMode()` helper at src-assignment
  only, so the re-navigation guard (which compares pathnames) still matches and a
  dark-mode toggle does not reload the example. Omitted when `suppressDarkMode`.
- Apply it synchronously in `example-runner.js` `setUpPage()`, before any example
  module runs.
- Make the generator-injected chart-theme snippet read the theme mode at apply
  time and wait for it, register its `color-scheme-change` listener before the
  initial apply, and no-op when the theme list is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* AG-18256 - Pin the generator dark-mode snippet's apply-time contract

The typescript and javascript bodies of the injected chart dark-mode code are
maintained separately, so a change applied to only one is a silent half-fix
across frameworks. Assert both read the theme mode at apply time, wait for it
alongside the grid api, register the listener before the initial apply, and skip
an unchanged theme list.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* AG-18256 - Keep the light-mode fallback and re-point in-flight example navigations

* AG-18256 - Cover the light-mode fallback and the in-flight navigation re-point

* AG-18256 - Format

* AG-18256 - Cover the example url's colour scheme with a docs e2e regression test

* AG-18256 Chart themes follow the grid's colour scheme

Integrated charts render to a canvas, so they cannot inherit the grid's
colour scheme from CSS. Without chartThemes the stock light themes were
used regardless, leaving a dark grid beside a light chart.

The scheme is resolved by painting --ag-background-color onto an
off-screen probe and reading the computed value back, because the
theming API emits that variable as authored (hsl(), color-mix(), var()
chains) and it is not readable as text.

The example spec now samples the rendered chart canvas rather than the
chartThemes option, which the example's own dark-mode code sets.

* AG-18256 - Format

* AG-18256 - Revert the chart-theme colour-scheme change to grid code

Out of scope for a website regression ticket. The docs fix stands on its
own: the runner applies the colour scheme before the entry module runs and
the generated example snippet sets the dark chart themes itself.

Tracked separately as an improvement to the grid.

* AG-18256 - Remove the explanatory comments

---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: sdwvit <victor.musienko@ag-grid.com>
Co-authored-by: Victor Musienko <8777372+sdwvit@users.noreply.github.com>
(cherry picked from commit 6a81bdd)