fix: mark QueryFallbackChain public API methods with @code_is_used (#993)

The dead code detector flagged four methods as unused, but they are
part of the intentional public API. These methods provide essential
introspection capabilities for logging, debugging, and observability.

- get_attempts(): Returns full attempt history for analysis
- get_attempt_summary(): Provides human-readable summaries for logs
- has_attempts(): Checks if chain has been executed
- was_successful(): Checks if any strategy succeeded

All four methods are extensively tested and documented in the class
docstring. They were flagged only because runtime tracing doesn't
exercise them in CLI paths, but they are used in tests and may be
needed for future logging/debugging features.

Co-authored-by: florath-ai-assistant[bot] <Andreas.Florath@telekom.de>

Author: coding-ai-assistant[bot]

src/aletheia_probe/fallback_chain.py

@@ -4,6 +4,8 @@
 
 from pydantic import BaseModel, Field
 
+from .utils.dead_code import code_is_used
+
 
 class FallbackStrategy(Enum):
     """Standard fallback strategies used across backends."""
@@ -91,10 +93,12 @@ def get_successful_strategy(self) -> FallbackStrategy | None:
                 return attempt.strategy
         return None
 
+    @code_is_used  # Part of public API, used in tests, needed for observability
     def get_attempts(self) -> list[FallbackAttempt]:
         """Return all logged attempts."""
         return self.attempts.copy()
 
+    @code_is_used  # Part of public API, used in tests, needed for logging/debugging
     def get_attempt_summary(self) -> str:
         """Return human-readable summary of attempts.
 
@@ -109,10 +113,12 @@ def get_attempt_summary(self) -> str:
             parts.append(f"{attempt.strategy.value}({status})")
         return " → ".join(parts) if parts else "no attempts"
 
+    @code_is_used  # Part of public API, used in tests
     def has_attempts(self) -> bool:
         """Check if any attempts were logged."""
         return len(self.attempts) > 0
 
+    @code_is_used  # Part of public API, used in tests
     def was_successful(self) -> bool:
         """Check if any attempt succeeded."""
         return self.get_successful_strategy() is not None