fix: correctness and hygiene fixes on released simplicityhl 0.6.0

[KyrylR]

1. Optimized (reduced features) tokio
2. Reduced binary size (builde time increased)
3. find_function_call_context and extract_function_name scanned with a character counter but sliced the line by byte offset (line.len() - 1 - i).
4. fixed did_change indexed content_changes[0], which panics on the empty change
     list some clients send
5. did_close was an empty stub, so a closed file kept its diagnostics displayed in the editor
6. on_change analysed without the lock and then wrote unconditionally.

Hygiene:

• Removed the unused LspError::DocumentNotFound variant. Code 4 is left unallocated so the remaining error codes stay stable.

[gerau]

ACK a3c251e; code review