Fix RBAC denial message format and conditional HelpTool disclaimer

[MRiabov]

• Removed "Error: " prefix from RBAC denial messages in src/tool_executor.py to match spec.
• Implemented conditional disclaimer in src/services/chat/handlers/idle.py to only append role-access disclaimer for restricted topics (invoice, export, etc.) when user is not OWNER.
• Added regression tests in tests/test_rbac_refinements.py.

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PR created automatically by Jules for task 7389735674475020579 started by @MRiabov

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