fix(streaming): chat and lessons return SSE errors instead of servlet 500s

[WilliamAGH]

Summary

Chat and guided learning now keep dependency failures inside the SSE contract instead of surfacing servlet HTTP 500s. The dev deployment also keeps LLM gateway traffic on the production tier even when the tier env var is blank.

Changes

Bug Fixes

• Chat requests no longer crash as servlet 500s when retrieval or reranking fails before the first token: prompt setup now runs inside the returned stream and maps setup/stream failures to structured SSE error events (ChatController.stream).
• Guided lesson chat no longer crashes as servlet 500s when lesson retrieval or reranking fails before streaming begins: guided prompt setup now runs inside the returned stream and emits structured SSE errors (GuidedLearningController.streamGuidedResponse).
• Guided lesson pages now stream lesson content through the same SSE parser as chat instead of blocking on JSON: /api/guided/content/stream emits JSON-wrapped text/error events, and the Learn view consumes that stream with abort protection (GuidedLearningController.streamLesson, streamLessonContent, LearnView.svelte).
• A transient primary-provider backoff no longer hides the only configured LLM gateway provider: provider routing only removes a backed-off primary when an alternate provider is actually configured (OpenAiProviderRoutingService.selectAvailableProviderCandidates).
• LLM gateway calls no longer risk an empty capacity tier header from blank env config: blank LLM_GATEWAY_TIER resolves to production-z, preserving the higher-capacity queue contract (OpenAIStreamingService.createClient).

Verification

• frontend/node_modules/.bin/svelte-check --tsconfig ./tsconfig.json
• frontend/node_modules/.bin/vitest run
• ./gradlew test
• ./gradlew build
• Pre-push hook: frontend build, backend build, full test suite, lint, and frontend check passed.
• Dogfood on https://dev.javachat.ai at 79ce02319aef3bd33f004c0156aa6e4970a3350e: browser load, main chat stream, guided lesson content stream, and guided lesson chat stream all passed with no console errors and no CustomErrorController 500s in the container logs.

Breaking Changes

None

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 98de0e67-a88d-4c15-b6a1-29085075858e

📥 Commits

Reviewing files that changed from the base of the PR and between 8a4e1fd and 79ce023.

📒 Files selected for processing (12)

• frontend/src/lib/components/LearnView.svelte
• frontend/src/lib/components/LearnView.test.ts
• frontend/src/lib/services/guided.test.ts
• frontend/src/lib/services/guided.ts
• frontend/src/lib/services/sse.ts
• src/main/java/com/williamcallahan/javachat/service/GuidedLearningService.java
• src/main/java/com/williamcallahan/javachat/service/OpenAIStreamingService.java
• src/main/java/com/williamcallahan/javachat/service/OpenAiProviderRoutingService.java
• src/main/java/com/williamcallahan/javachat/web/ChatController.java
• src/main/java/com/williamcallahan/javachat/web/GuidedLearningController.java
• src/test/java/com/williamcallahan/javachat/service/OpenAIStreamingServiceTest.java
• src/test/java/com/williamcallahan/javachat/web/GuidedSseCitationEventTest.java

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dev

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