fix: improve media failure diagnostics and token fallback coverage

tests/utils/test_token_estimation.py

@@ -0,0 +1,32 @@
+from nanobot.utils.helpers import estimate_prompt_tokens_chain
+
+
+class _NoCounterProvider:
+    pass
+
+
+class _BrokenCounterProvider:
+    def estimate_prompt_tokens(self, messages, tools=None, model=None):
+        raise RuntimeError("counter unavailable")
+
+
+def test_estimate_prompt_tokens_chain_falls_back_without_provider_counter() -> None:
+    tokens, source = estimate_prompt_tokens_chain(
+        _NoCounterProvider(),
+        "test-model",
+        [{"role": "user", "content": "hello"}],
+    )
+
+    assert tokens > 0
+    assert source == "tiktoken"
+
+
+def test_estimate_prompt_tokens_chain_falls_back_when_provider_counter_fails() -> None:
+    tokens, source = estimate_prompt_tokens_chain(
+        _BrokenCounterProvider(),
+        "test-model",
+        [{"role": "user", "content": "hello"}],
+    )
+
+    assert tokens > 0
+    assert source == "tiktoken"