buffer, worker: resolve error handling TODOs

[dahiya001rohit]

This PR resolves several TODO comments related to error handling in
lib/buffer.js
and
src/node_worker.cc
to improve error reporting and debuggability.

Changes:

lib/buffer.js
:
Buffer.concat: Now throws ERR_INVALID_ARG_VALUE instead of ERR_INVALID_ARG_TYPE when a list element is not a Buffer or Uint8Array. This provides a clearer error message indicating the specific invalid value.
Buffer.byteLength: Now throws ERR_UNKNOWN_ENCODING when an unknown encoding is provided, instead of silently falling back to UTF-8.
Note: This is a semantic change. Previously, unknown encodings would default to UTF-8. This change enforces valid encodings as suggested by the code comments.
src/node_worker.cc
:
Updated
WorkerThreadData
initialization to use ExitCode::kBootstrapFailure instead of ExitCode::kGenericUserError for uv_loop_init and NewIsolate failures.
Updated Worker::NearHeapLimit to use ExitCode::kV8FatalError instead of ExitCode::kGenericUserError for out-of-memory situations.
Updated Worker::Run to use ExitCode::kBootstrapFailure for NewContext failures.
Tests:

Updated
test/parallel/test-buffer-concat.js
to expect ERR_INVALID_ARG_VALUE.
Updated
test/parallel/test-buffer-bytelength.js
to expect ERR_UNKNOWN_ENCODING.
Verified that make -j4 node builds successfully and tests pass.
Labels: buffer, worker, semver-major (due to Buffer.byteLength throwing on unknown encoding)