support LSP `textDocument/inlineCompletion`

## Problem

Nvim 0.12 gained support for LSP `textDocument/inlineCompletion` https://github.com/neovim/neovim/pull/33972

## Expected behavior

amazonq.nvim can support this by adding an explicit handler , similar to what we did for `'textDocument/completion'` https://github.com/awslabs/amazonq.nvim/blob/9e912b9d59ca6f0e75086524b7289db2282799f5/lua/amazonq/completion.lua#L30-L31

Alternatively, the "ideal" approach would be to drop that entire in-process-LSP-server in favor of upstream support in the [Q LSP server](https://github.com/aws/language-servers/blob/ef7d7931954f5083e4a5c358e67c6dc652fa1a40/server/aws-lsp-codewhisperer/README.md). But that doesn't need to block this.

Start with the existing textDocument/completion handler in lua/amazonq/completion.lua and compare it with Neovim 0.12's textDocument/inlineCompletion support. Review the linked Q LSP documentation, then add explicit inline-completion handling so amazonq.nvim supports the new request.

Written by the indexing model from the issue text.