fix: harden Rust playground validation limits

[emirykl]

Summary

• enforce a 64 KiB UTF-8 source limit and return structured HTTP 413 responses
• bound validation to 100 ms and 100 diagnostics with actionable status and diagnostic codes
• preserve normal validation behavior while preventing internal exception details from leaking
• add focused service and API tests for normal code and limit failures

Test evidence

• npm test -- --no-watchman --runTestsByPath tests/playground.validation.test.ts — passed, 9/9
• strict isolated TypeScript check for the changed service and route — passed
• git diff --check — passed
• full backend Jest run: 28 suites passed; 14 unrelated baseline suites failed due existing queue declaration, Redis, SEO, and environment-test issues
• full tsc --noEmit: existing unrelated Redis, webhook, storage, and notification errors remain

Fixes #922

Verification

• PASS: Focused Jest suite — 9/9 tests
• PASS: Strict TypeScript check for changed service and route
• PASS: git diff --check
• BASELINE: Full backend Jest — 28 suites passed, 14 unrelated suites failed
• BASELINE: Full TypeScript check has existing unrelated Redis, webhook, storage, and notification errors

Diagnostics

Recovered command failures: /bin/zsh -lc 'npm test -- --no-watchman --runTestsByPath tests/playground.validation.test.ts' (127), /bin/zsh -lc 'npm ci --ignore-scripts' (255), /bin/zsh -lc 'npm_config_cache=/private/tmp/web3-student-lab-npm-cache npm ci --ignore-scripts' (1), /bin/zsh -lc "rg -c 'cdn\\.npmmirror\\.com' package-lock.json && rg -n 'cdn\\.npmmirror\\.com' package-lock.json | head -n 20" (1), /bin/zsh -lc 'npx tsc --noEmit' (2), /bin/zsh -lc 'npx prisma generate --schema=prisma/schema.prisma' (1), /bin/zsh -lc 'npx tsc --noEmit' (2), /bin/zsh -lc 'npm test -- --no-watchman' (1)

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