fix(desktop): add OAuth account channels to Models story bridge

[AlvaroBalbin]

The Models settings page story rendered against a bridge with no subscription channels, so ModelOAuthSection's experimental-gate call rejected on mount: the Claude card never appeared and the other three stayed frozen at their static 可用 label, a screen the app never shows. I added claudeSubscription (including isExperimentalEnabled), openAiCodex, githubCopilotSubscription and xaiOAuth to that story bridge, plus a contract assertion that derives the required channel list from provider-oauth-section.tsx so a newly added channel fails loudly instead of quietly degrading the story. The remaining visual audit in the issue is left for after #1421 lands, since it rewrites the same form and fixtures. Fixes #1367.

I pushed this to my fork first and the project's own CI workflows pass on the commit.

[Astro-Han]

Reviewed at 2f7b911b0e3596c5c1c9cb1b1ea45ef7e18c4233.

No P0-P2 findings.

P3: The Story fixtures do not satisfy the full preload return types.

All four account states omit provider, and the Claude profile omits accountUuid. The contract test only checks for top-level channel names, so it would still pass if a getAccountState method were removed or rejected. This does not break the current Story rendering, so it did not block the merge. A follow-up should type the fixtures from the real return types and add play assertions for the four cards and the expected signed-in states.

Squash-merged as 19188a32a2600605b95b494ce979910e9bdaac1a.