feat: add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud to the built-in third-party provider registry
• configure its OpenAI-compatible endpoint and default nested model ID
• cover session routing through the existing third-party chat path

Verification

• npx vitest run src/utils/chatApiRoute.test.ts (2 passed)
• npx eslint src/types/settings.ts src/utils/thirdPartyApiProviders.ts src/utils/chatApiRoute.test.ts --max-warnings=0
• npx prettier --check src/types/settings.ts src/utils/thirdPartyApiProviders.ts src/utils/chatApiRoute.test.ts
• npm run build
• npm run build:api
• live model discovery returned 123 Atlas Cloud models; the default model was present and a chat request through sendOpenAICompatibleMessageNonStream returned content

Existing baseline issues

• npm run typecheck still reports the existing LanguageIcon.tsx:218 compactLabel error
• the full Vitest run still has three existing useAppEvents.test.tsx Tab-cycle failures; all three reproduce on the unmodified base commit

Risk

• the new provider is disabled by default and reuses the existing OpenAI-compatible model discovery, connection test, and chat request paths.

[binyangzhu000-sudo]

Rebased onto current main. This turned out to be more than a conflict fix: main replaced the fixed-id provider record (providers keyed by id + activeProvider) with a list of connections created from templates, so the original implementation no longer fit. Commit 0abef0c adapts it.

What changed:

• Moved atlascloud out of LEGACY_THIRD_PARTY_PROVIDER_IDS and into THIRD_PARTY_TEMPLATE_IDS. Atlas Cloud never shipped in a released version, so there are no stored settings to migrate — and leaving it in the legacy list would have left LEGACY_TEMPLATE_ID and THIRD_PARTY_PROVIDER_LABELS without entries for it. Worth flagging because a plain textual merge does resolve cleanly here and silently puts it in the legacy list.
• Described the provider through TEMPLATE_DEFAULTS (name, base URL, default model, protocol, API-key and docs links) so it can be created from the add-connection dialog.
• Filled in the remaining per-template records: THIRD_PARTY_TEMPLATE_LABELS, TEMPLATE_LABEL_KEYS plus its i18n entry, and THIRD_PARTY_TEMPLATE_LOGO — reusing the generic logo there, since there is no Atlas Cloud icon asset in assets/model-icons. Happy to add one if you'd prefer a dedicated icon.
• Rewrote the routing test against the connection model, keeping every test main added in the meantime.

Validation:

• vitest run — 466 files, 3280 tests passed.
• eslint --max-warnings=0 on the six touched files — clean.
• tsc --noEmit reports 4 errors (TextEditorModal.tsx ×2, GenerationSection.tsx, standardChatApiCall.ts). These are pre-existing: I ran the same check on a clean origin/main worktree and got the identical 4. None are in files this PR touches.

The net PR diff is 6 files / +54. CI currently shows action_required, which needs a maintainer to approve the workflow run for a fork.