[codex] Make LSP server pass strict mypy

[jsiek]

Summary

• type the custom-request parameter helpers in lsp/lsp_server.py
• reuse helpers for LSP range/workspace-edit payloads
• add lsp.lsp_server to the strict mypy override block without untyped-decorator enforcement, matching optional pygls behavior in CI

Validation

• make tests
• python3 -m pytest test/lsp/test_lsp_server.py
• python3 -m mypy --strict lsp/lsp_server.py --follow-imports=silent
• python3 -m mypy .
• python3 -m ruff check lsp/lsp_server.py pyproject.toml
• git diff --check

Progresses #506.