Handle newer Vega-Lite MIME types from Altair 6+

[SAKSHI-DOT693]

Altair 6 introduces a new Vega-Lite MIME type (application/vnd.vegalite.v6+json).
FigureAltair currently assumes only v4/v5 MIME variants, which leads to a
KeyError and a full UI crash when charts are rendered with newer Altair
versions.

This change preserves the existing v4/v5 code path and adds a controlled
fallback for newer Vega-Lite MIME types by selecting the first available
Vega-Lite spec exposed by Altair. The intent is to maintain backward
compatibility while avoiding crashes caused by forward MIME version changes.

Notes

No behavior change for Altair ≤ 5

Prevents hard failures with newer Vega-Lite MIME variants

Scoped strictly to Vega-Lite rendering
No behavior change for Altair ≤ 5

Prevents hard failures with newer Vega-Lite MIME variants

Scoped strictly to Vega-Lite rendering

[EwoutH]

@maartenbreddels could you review this PR?

If this would fix #1127 that would be really useful

[SAKSHI-DOT693]

Thanks for flagging this and for tagging @maartenbreddels.

This PR focuses specifically on making the Vega-Lite MIME handling more robust for newer Altair versions, by avoiding hard failures when newer MIME types (e.g. v6+) are emitted.

This overlaps with the behavior described in #1127, where the frontend encounters unexpected Vega-Lite state or MIME keys during the initial render.

If you’d prefer this PR to explicitly reference or add coverage for #1127 (e.g. via a small targeted test or adjustment), I’m happy to extend it — just let me know the preferred direction.

[SAKSHI-DOT693]

I’ve tested this locally on Windows with Altair installed. The updated FigureAltair logic now renders successfully and gracefully selects the available Vega-Lite MIME spec without raising errors. No regressions observed in the existing behavior.