feat(ai): add Atlas Cloud chat provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud chat client on top of the existing OpenAI-compatible runtime
• support ATLASCLOUD_* and ATLAS_CLOUD_* API key/base URL environment aliases
• register deepseek-ai/deepseek-v4-pro and qwen/qwen3.5-flash as opt-in chat candidates without changing the default tiers
• add trace display names and focused provider/routing coverage

Validation

• ./mvnw -pl bootstrap -am -DskipTests spotless:check
• ./mvnw -pl bootstrap -am -Dtest=ModelSelectorTest,AtlasCloudChatClientTest -Dsurefire.failIfNoSpecifiedTests=false test (11 passed)
• ./mvnw -pl bootstrap -am -DskipTests package
• npm ci --ignore-scripts and npm run build in frontend
• parsed application.yaml successfully
• verified both model IDs against the live Atlas Cloud /v1/models catalog (121 models)
• git diff --check

npx eslint src/pages/admin/traces/traceUtils.ts --max-warnings 0 is currently blocked before linting by the repository's existing ESLint 8 / react-refresh config mismatch (Unexpected top-level property "name").

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