fix(invoice): prevent None agent_notes writing literal string to DB

[sshekhar-04]

Problem

Fixes #139

When update_invoice_status and update_invoice_agent_notes are
called with agent_notes=None, f-string interpolation writes the
literal string "\n\nNone" into the invoice audit trail.

This pollutes forensic records and causes CTF detectors to produce
false positives.

Fix

Added or '' guard with .strip() on agent_notes before
f-string interpolation in both affected functions.

Before:

new_notes = f"{existing_notes}\n\n{agent_notes}"

After:

new_notes = f"{existing_notes}\n\n{agent_notes or ''}".strip()

Files Changed

• finbot/tools/data/invoice.py — fixed 2 occurrences
• tests/unit/tools/test_invoice.py — added 6 tests

Tests ✅

All 6 passing.

Closes #139