[18.0][FIX] report_py3o: use py3o report in invoice Send flow

[fsmw]

Summary

• Override the report_py3o pre-render path for py3o reports when the invoice Send flow calls _pre_render_qweb_pdf.
• Generate per-record PDF streams compatible with the account move send pipeline.
• Raise an explicit ValidationError when py3o output is not PDF in that flow.
• Add regression tests for stream generation and non-PDF guard.

Why

In invoice Send plus Download flow, account calls _pre_render_qweb_pdf directly.
For py3o reports this bypassed py3o rendering and defaulted to qweb behavior.

Reproduction

• Configure an invoice report with report_type set to py3o.
• Open invoice Send wizard and click Download.
• Before fix: qweb template is used.
• After fix: py3o report is rendered.

Tests

• report_py3o suite on db test_1144_report_py3o.
• Result: 0 failed, 0 errors.

Closes #1144