feat: actionable error + APP_MEMORY_MB recommendation when a report exceeds the JSON string cap

[NotYuSheng]

Summary

Follow-up to #793/#792. Even with the raised cap, a report can still exceed it (or exceed the pre-raise default on an unmigrated deployment) — and today that fails as a generic 500 "Report generation failed. Please try again.", with no indication of why or what to do.

• Backend (GlobalExceptionHandler): handles HttpMessageNotReadableException directly instead of letting it fall into the catch-all. When the cause is a StreamConstraintsException from the string-length cap, parses the attempted size out of Jackson's own exception message and computes a recommended APP_MEMORY_MB by inverting docker-entrypoint.sh's derivation formula (divisor 40, clamp 256MB) — with a distinct message when the request is past that clamp entirely, since no amount of APP_MEMORY_MB would help there. Other malformed-JSON-body cases (unrelated to this cap) now get a proper 400 instead of the same 500 catch-all they silently fell into before.
• Frontend (AnalysisPage.tsx): reads the structured error and shows an actionable Alert with the attempted size and recommended value, instead of the generic failure text. Had to account for the report request's responseType: 'blob' applying to error bodies too — the error JSON comes back as a Blob, not a parsed object, and has to be read out explicitly. Follows this repo's existing CONTEXT_LENGTH_EXCEEDED pattern (LLM story generator) for structured, actionable error UX.

Test plan

• GlobalExceptionHandlerTest (new cases): unrelated malformed body → 400; string-length cap → 413 with PAYLOAD_STRING_TOO_LARGE; recommendation math verified against the real reported failure (20,054,016 chars → recommends 1024MB, and independently re-derives that 1024/40 ≥ the attempted size); past-the-hard-ceiling case → recommendedAppMemoryMb is null with the right message. mvn test — 473/474 passing (the one failure is a pre-existing local-environment gap, missing tshark, unrelated to this change).
• npx tsc --noEmit — clean. npm run build — clean.
• New Playwright e2e test (llm-error.spec.ts) forcing the 413 via route interception, run against a real rebuilt stack in a real browser — alert renders and is visible with the attempted size and recommended value. (This file specifically exists to catch SGDS Alert rendering null under React 19 when defaultProps aren't restored — real value in testing the actual DOM here, not just TypeScript types.)
• End-to-end against the real backend (not mocked): rebuilt the stack, POSTed an oversized payload to /api/v1/files/{fileId}/report — got back 413 with attemptedSizeMb: 52, recommendedAppMemoryMb: 2560, confirming the real Jackson exception flows through this new handler correctly.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QDyaaPSRNzNXRzJoS794C4

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