fix(text-splitters): remove incorrect C# and Elixir separator keywords (#37037)

## Summary

Removes two incorrect separators from `get_separators_for_language()` in
`RecursiveCharacterTextSplitter`:

- **C#**: `"\nimplements "` is a Java keyword. C# uses `:` for interface
implementation. This separator never matches valid C# source code.
- **Elixir**: `"\nwhile "` does not exist in Elixir. The language uses
recursion and `Enum.reduce_while/3` instead of while loops.

Both are dead separators that silently degrade chunking quality by
occupying positions in the separator priority list without contributing
useful split points.

## Tests

Added two targeted tests:
- `test_csharp_separators_no_java_keywords`: verifies `"\nimplements "`
is not in the C# separator list
- `test_elixir_separators_no_while`: verifies `"\nwhile "` is not in the
Elixir separator list

Existing `test_csharp_code_splitter` continues to pass (no change to
expected output since `implements` never matched valid C# code).

Full suite: 129 passed, 0 failed.

Fixes #37030

libs/text-splitters/langchain_text_splitters/character.py

@@ -440,7 +440,6 @@ class RecursiveCharacterTextSplitter(TextSplitter):
                 # Split along control flow statements
                 "\nif ",
                 "\nunless ",
-                "\nwhile ",
                 "\ncase ",
                 "\ncond ",
                 "\nwith ",
@@ -593,7 +592,6 @@ class RecursiveCharacterTextSplitter(TextSplitter):
             return [
                 "\ninterface ",
                 "\nenum ",
-                "\nimplements ",
                 "\ndelegate ",
                 "\nevent ",
                 # Split along class definitions