[19.0][MIG] calendar_export_ics: Migration to 19.0

[tishmen]

Scope

• calendar_export_ics only

Summary

• Migrate to Odoo 19 using the working feature branch code.
• Context handling aligned with 19.0.
• Asset paths are relative under web.assets_backend.

Pre-commit

• Verified locally; no additional changes applied here to keep parity with the feature branch.

Tests Command:

./odoo/odoo-bin -c odoo.conf -d odoo_calendar_tests_export_ics_001 \
  -i calendar_export_ics \
  --test-enable --test-tags "/calendar_export_ics" \
  --log-level=info --stop-after-init

Result: 3 tests green locally.

[tishmen]

Supersedes #179 to preserve history per OCA 19.0 migration process.