Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tests/test_reasoning_param.py

@@ -17,7 +17,7 @@ def _body(reasoning_max_tokens, api_mode="chat", max_tokens=1000):
 
 
 def test_zero_sends_nothing():
-    """Ollama and friends reject unknown fields — 0 must stay silent."""
+    """Ollama and other providers reject unknown fields. Sending 0 must not add a `reasoning` field."""
     assert "reasoning" not in _body(0)
 
 
@@ -36,7 +36,7 @@ def test_negative_turns_reasoning_off():
 
 
 def test_off_is_not_merely_excluded():
-    """`exclude: true` still thinks and still bills; we want it actually off."""
+    """`exclude: true` still generates and bills for reasoning tokens. The off setting must omit the field entirely instead of relying on `exclude`."""
     assert _body(-1)["reasoning"].get("exclude") is None