fix(chat): include conversation history in Gemini prompt

[Tejas-h-blitz]

🐛 Problem

Conversation history sent by the client was never included in the prompt
sent to Gemini. The prompt construction used an if/else if chain where
SAFETY_INSTRUCTION (a module-level constant) is always truthy, so the
historyBlock branch never executed. Every message was treated as the
first message in a new conversation.

Closes #645

📝 What does this PR do?

Replaces the broken if/else if chain with a single prompt builder that
always includes both SAFETY_INSTRUCTION and historyBlock (when
present):

const prompt = [
  SYSTEM_PROMPT,
  SAFETY_INSTRUCTION,
  historyBlock ? `Conversation so far:\n${historyBlock}` : "",
  `[USER INPUT START]\n${sanitized}\n[USER INPUT END]`,
]
  .filter(Boolean)
  .join("\n\n");

Also removed the redundant typeof chatSchema !== 'undefined' guard
since chatSchema is always defined in this file.

🧪 How was this tested?

• Sent multi-turn messages and verified Gemini response references prior
  context
• Verified single-message (no history) requests still work correctly
• Verified prompt injection sanitization still applies

✅ Checklist

• I've read the CONTRIBUTING guide
• My code follows the project's style guidelines
• I've tested my changes locally
• I've linked the related issue
• I haven't introduced any new secrets or API keys