lib/types: fix toCoerced's typeMerge

[HexoKnight]

Description of changes

Fixes lib.types.coercedTo's typeMerge which previously (for 8 years!) took 2 arguments when it should only take 1.

This didn't actually affect functionality as coercedTo cannot merge anyway but it results in an awful error message: error: value is a function while a set was expected because mergeOptionDecls then thinks the types are actually mergeable (as the partially applied function is != null) leading to a worse error message down the line as type is not an attrset.

I've added a test (mainly as a reproducible example) but let me know if it's unnecessary/overkill.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[infinisil]

Oh my, thank you! Also very nice to have a test for this :D