fix(vim): pass position encoding to Neovim - #4532

Summary

• fetch the active Tabby LSP client before building inline-completion parameters
• pass that client offset encoding to vim.lsp.util.make_position_params
• return early when the client is no longer available

This removes the Neovim 0.11 position-encoding warning and avoids selecting the encoding of an unrelated first client when several LSP clients are attached.

Test plan

• luac -p clients/vim/lua/tabby/lsp/nvim_lsp.lua
• isolated Lua regression scenarios for an active and a missing client
• verified with Neovim 0.10.4 and 0.11.7

Fixes #4527