Add input validation limits for conversation history in /chat endpoint

[Copilot]

Addresses security concern where unbounded conversation history could cause memory exhaustion or excessive API costs.

Changes

• Message count limit: Reject requests with more than MAX_MESSAGES (default: 50)
• Content length limit: Reject requests exceeding MAX_CONTENT_LENGTH characters (default: 100,000)
• Safe env parsing: Helper function _safe_int_env() handles invalid env values gracefully
• Early termination: Content length validation exits loop once limit exceeded

Both limits are configurable via environment variables.

Example response on limit exceeded

{
  "error": "Too many messages. Maximum allowed is 50."
}

{
  "error": "Content too long. Maximum allowed is 100000 characters."
}

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