fix: prevent crash when encoding payload with invalid UTF-8

[agity7]

Summary

This PR wraps vim.fn.json_encode() in a pcall() to avoid crashing when Codeium tries to encode invalid UTF-8 payloads.

Problem

Some buffers (e.g., minified .mjs, .sum, or binary-like files) contain invalid UTF-8. Codeium tries to encode the completion request payload with vim.fn.json_encode, leading to:

Vim:E474: String contains byte that does not start any UTF-8 character

Fix

• Wraps the encoding with pcall.
• Logs a meaningful error and returns early if encoding fails.

Tested locally with such files, and Codeium no longer crashes.

Fixes a class of bugs seen when using preview panels or telescope buffers as well.

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