Fix vim.validate deprecation warnings in telescope.actions.utils

[Trishthedish]

Description

This PR addresses deprecation warnings caused by the old, table-based vim.validate syntax in two functions within telescope.actions.utils. The validation calls are updated to use the new argument-based syntax recommended by Neovim.

While using Telescope (master branch at commit b4da76b), I began seeing repeated deprecation warnings related to calls like vim.validate({}).

At the time I began working on this, there wasn’t an open issue for it, so I decided to start addressing the problem incrementally — beginning with the two usages in telescope.actions.utils.

Since then, issue #3499 has been opened describing the same deprecation warnings. I’ve linked this PR to that issue for tracking purposes.

Example warning:

WARNING vim.validate is deprecated. Feature will be removed in Nvim 1.0
  ADVICE:
      - use vim.validate(name, value, validator, optional_or_msg) instead.

Motivation and Context

One of my 2025 goals is to contribute meaningfully to an open-source Neovim plugin I use regularly. I’ve been looking for small, self-contained improvements that would let me practice writing and testing Lua code in the Neovim ecosystem — ideally in a way that supports test-driven development approach.

This change gave me an opportunity to explore Neovim plugin internals while also getting some hands-on experience writing tests before modifying behavior, which helped build confidence in the refactor.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added unit tests for map_entries and map_selections
Located in tests/automated/actions/utils_spec.lua
Covers:

• Input validation (e.g. nil, wrong types)
• Edge cases (empty or malformed inputs)
• Row/index mapping per Telescope conventions
• Integration scenarios with realistic selections
• Performance (1000+ entries, 500+ selections)
• TDD-style validation tests for both functions that only pass with the new vim.validate syntax

Reproduction Instructions:

Clone branch: git checkout feat/improve-actions-utils-validation
To run only newly added tests use:

make test-utils

(Temporary Makefile helper I added for local dev; happy to remove before merge)

Configuration:

• Neovim version: NVIM v0.11.0 Build type: Release LuaJIT 2.1.1744318430
• Operating system and version: macOS Sequoia 15.5 (Apple Silicon - M2)

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation (lua annotations)

N/A – no new public API or parameter changes