fix(test-infra): remove redundant ${PYTHON} variable definitions from robot files

Remove the local ${PYTHON}    python (and python3) variable definitions from
the *** Variables *** sections of all affected robot files. These local
definitions were overriding the pabot-injected venv Python path passed via
--variable PYTHON:/path/to/venv/python, causing tests to use the system
Python (which lacks required packages like structlog, sqlalchemy, etc.)
instead of the nox venv Python.

The correct ${PYTHON} value is already set by Setup Test Environment in
common.resource via sys.executable, and pabot passes it via --variable.
The local fallback definitions are redundant and harmful in parallel runs.

Audit found 56 robot files with the pattern (more than the 9 originally
identified in the issue). All occurrences have been removed.

ISSUES CLOSED: #1309

robot/resource_cli.robot

@@ -7,7 +7,6 @@ Suite Teardown    Clean Up Test Database
 Test Tags         resource_cli
 
 *** Variables ***
-${PYTHON}         python
 ${DB_URL}         sqlite:///build/test_resource_cli.db
 
 *** Keywords ***