fix(reka-ai): guard first-message access in chat transform

[ankit25bcs10610]

What

The conversation_history transform in RekaAIChatCompleteConfig dereferences messages[0] (a local accumulator array) before anything is pushed to it:

const messages: RekaMessageItem[] = [];
...
if (media_url && messages[0].media_url) { ... }  // messages still empty
...
if (messages[0].type !== 'human') { ... }        // empty / fully-filtered list

• The first check throws TypeError: Cannot read properties of undefined when the first message is an image (media_url truthy, messages still empty).
• The second throws when the resulting message list is empty (e.g. no messages, or all filtered out).

Fix

Optional-chain both accesses (messages[0]?.media_url, messages[0]?.type), so an image-first or empty message list is handled gracefully.

Tests

Added src/providers/reka-ai/chatComplete.test.ts covering both cases. Verified each fails before the fix (throws) and passes after.

npm run format:check passes.