[lukaszsamson]

Problem

Third-party Mix compilers can return plain strings (or other terms) in the diagnostics list instead of Mix.Task.Compiler.Diagnostic structs. Mix does not enforce the struct contract, so Build.run_mix_compile/1 passed these straight into Diagnostics.from_mix_task_compiler_diagnostic/3, which only matched the struct. The build process crashed and the user got a generic build error instead of the compiler's message.

Crash report (Elixir 1.20.0):

** (FunctionClauseError) no function clause matching in ElixirLS.LanguageServer.Diagnostics.from_mix_task_compiler_diagnostic/3
  language_server/diagnostics.ex:23: ElixirLS.LanguageServer.Diagnostics.from_mix_task_compiler_diagnostic("Make failed with exit code 2", "/path/mix.exs", "/path")
  Enum.map/2
  language_server/build.ex:200: ElixirLS.LanguageServer.Build.handle_compile_phase/6
  language_server/build.ex:20: anonymous fn/3 in ElixirLS.LanguageServer.Build.build/3

Fix

• Binary diagnostics become error diagnostics attached to mix.exs (position 0, compiler_name: "Mix"), matching how mixfile-level errors are attached.
• Any other term is passed through inspect/1 and reported the same way.
• Struct clause is unchanged.

Tests

Added two cases to apps/language_server/test/diagnostics_test.exs. mix test test/diagnostics_test.exs: 20 passed.