Fix(subprocess): remove tree-sitter dependency

[archie-judd]

Description

Neovim 0.11+ bundles tree sitter parsers in the runtime, and no longer requires nvim-treesitter for basic parsing. This PR updates the subprocess initialisation process to handle both cases.

Changes

• No longer search for, or add treesitter to the subprocess rtp.
• Discover parser directories via nvim_get_runtime_file("parser/*", true) and add their parents to rtp.
• Extract resolve_plugin_root() as a separate function for clarity.
• Rename add_to_rtp to add_paths_to_rtp with clearer signature
• Add regression test for parsing in subprocess.

Context

The subprocess needs parsers available for treesitter parsing of test files. Previously this relied on nvim-treesitter being installed. Now it works with:

• Neovim 0.11+ bundled parsers (no nvim-treesitter required)
• nvim-treesitter managed parsers (backwards compatibility)
• Both together

Fixes:

#531