Add upper bound validation for AI response length

[Copilot]

Addresses feedback from #144: the max_tokens increase to 800 lacked corresponding response length validation, allowing responses to exceed the "50-150 words" guidance.

Changes

• Added validation constants: MIN_AI_RESPONSE_LENGTH (20) and MAX_AI_RESPONSE_LENGTH (1000)
• Implemented max length check: Rejects responses >1000 chars (~150 words + 250 char buffer for markdown/links)
• Optimized validation: Single trim operation, reused across both min/max checks
• Improved observability: Both validation failures now log actual vs expected character counts

// Response validation constants
const MIN_AI_RESPONSE_LENGTH = 20;
const MAX_AI_RESPONSE_LENGTH = 1000; // ~150 words (~750 chars) + 250 char buffer for markdown

// Trim response once for validation
const trimmedResponse = aiResponse ? aiResponse.trim() : '';

if (!trimmedResponse || trimmedResponse.length < MIN_AI_RESPONSE_LENGTH) {
  console.log(`::warning::AI response too short (${trimmedResponse.length} chars, min ${MIN_AI_RESPONSE_LENGTH})`);
  // ... error handling
}

if (trimmedResponse.length > MAX_AI_RESPONSE_LENGTH) {
  console.log(`::warning::AI response too long (${trimmedResponse.length} chars, max ${MAX_AI_RESPONSE_LENGTH})`);
  // ... error handling
}

Responses exceeding the limit are rejected rather than truncated to maintain quality and enforce conciseness.

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