[lukaszsamson]

Problem

Completion crashes when a .beam is malformed or truncated (e.g. being rewritten by a concurrent compile). Code.fetch_docs/1 delegates to :beam_lib, which only catches throws, so the error escapes:

Exception truncated in report (Elixir 1.17.3 / 1.18.3, completion):
    (stdlib 6.2.2.2) beam_lib.erl:712: :beam_lib.read_info/1
    (elixir 1.18.3) Code.get_beam_and_path/1
    (elixir 1.18.3) Code.fetch_docs/1
    elixir_sense/core/normalized/code.ex:25: ElixirSense.Core.Normalized.Code.get_docs/2
    elixir_sense/core/introspection.ex:638: ElixirSense.Core.Introspection.get_module_docs_summary/2
    (elixir_ls_utils) ElixirLS.Utils.CompletionEngine.get_erlang_module_result/2  (also get_elixir_module_result/2)

Same root cause as #346 (Normalized.Typespec).

Changes

• Normalized.Code.fetch_docs/1 catches any raise/exit/throw from Code.fetch_docs and returns {:error, :chunk_not_found}. callback_documentation/1 now goes through this wrapper.
• ExCkReader on-disk :beam_lib.chunks/2 path catches the same and falls through to the existing binary fallback.
• Not changed: BuiltinFunctions Code.fetch_docs(Module) runs at compile time on a fixed module, so it cannot read an arbitrary beam at request time.

Tests

• Normalized.CodeTest: corrupt beam (byte flipped in the atom table) makes fetch_docs/1 and get_docs/2 return error/nil. Fails on master with the MatchError above.
• ExCkReaderTest: same corruption on the on-disk path; asserts read_chunk/2 returns a tagged result and does not raise. Fails on master.

mix test on the touched files: 45 passed. Full suite has the same 58 failures with and without this change (pre-existing ElixirTypes/BindingDescr property tests on the local 1.21-dev toolchain).