fix: avoid tracing-context AttributeError in service decorators

[AlessioSpallino]

Summary
This PR fixes a tracing-related crash in service decorators during function-call handling.

Problem
In some function-call paths, tracing context access could raise:
'NoneType' object has no attribute 'get_turn_context'
This can happen when _tracing_context is truthy at the call-site check, but the underlying getter path raises an AttributeError.

Root Cause
_get_turn_context() and _get_parent_service_context() used direct getter calls after a truthiness check.
That check does not protect against AttributeError raised inside get_turn_context() / get_conversation_context().

How to reproduce

• Disable tracing (enable_tracing=False).
• Trigger a tool/function call (runs through LLMService._run_function_call). Tracing decorator helpers can raise AttributeError: 'NoneType' object has no attribute 'get_turn_context' when tracing context isn’t available.

Changes
Updated _get_turn_context to return None on AttributeError.
Updated _get_parent_service_context to return None conversation context on AttributeError, then keep the existing fallback behaviour.

Tests
Added targeted regression tests in tests/test_service_tracing_decorators.py:

• test_get_turn_context_handles_none_context
• test_get_turn_context_handles_truthy_broken_proxy
• test_get_parent_service_context_handles_none_context
• test_get_parent_service_context_handles_truthy_broken_proxy

Validation run: uv run pytest tests/test_service_tracing_decorators.py