Fix(chat): resolve stale system-prompt date with explicit per-call evaluation

[Jean-Regis-M]

Summary

Fixes #442 stale date in system prompt for long-lived sessions crossing midnight.

_get_system_prompt() now captures current_date as an explicit local variable at method entry in both VendorChatAssistant and CoPilotAssistant, guaranteeing a fresh date on every call.

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Problem

datetime.now(UTC) was evaluated inline inside the f-string. While this works for single calls, it provides no structural guarantee of re-evaluation across conversation turns. Any future caching of the prompt string or a session spanning midnight would carry a stale date for the remainder of the day, producing off-by-one-day errors in financial reports and scheduling responses.

Root Cause

Implicit evaluation order: the date was computed inside the f-string with no contract enforcing re-evaluation per call.

# Before  date baked into f-string, no explicit re-evaluation guarantee
f"Today's date is {datetime.now(UTC).strftime('%Y-%m-%d')}."

Solution

Extract date computation into a named local variable at the top of _get_system_prompt() in both assistants, then reference it in the f-string.

# After  date captured explicitly at method entry, once per call
current_date = datetime.now(UTC).strftime("%Y-%m-%d")
f"Today's date is {current_date}."

This makes the evaluation point structurally explicit and immune to any prompt-string caching introduced later.

Files changed: vendor_chat_assistant.py, copilot_assistant.py 4 lines total.

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Impact

Area	Detail
Single calls	No behavioral change
Long-lived sessions	Date is now always fresh per turn
Breaking changes	None
Regression risk	Zero existing date-format tests pass unchanged

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Testing

pytest tests/unit/agents/test_chat_assistant.py::TestVendorSystemPrompt::test_chat_prompt_005_vendor_prompt_contains_current_date -v
pytest tests/unit/agents/test_chat_assistant.py::TestCoPilotPromptExtended::test_chat_prompt_040_copilot_prompt_date_format_is_iso -v

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Tasks

• Extract current_date local variable in VendorChatAssistant._get_system_prompt()
• Extract current_date local variable in CoPilotAssistant._get_system_prompt()
• Replace inline datetime.now() f-string expressions with {current_date}
• Verify existing date-format unit tests pass without modification