Add ${EXECUTION ID} automatic variable and --executionid option (#2743) - #5757

Implements #2743.

Adds a unique execution ID to each Robot Framework run.

What it does

• Exposes the ID as the ${EXECUTION ID} automatic variable.
• Stores it as Execution ID metadata on the top level suite, so it is visible in logs and reports and saved in output.xml (and carried over to rebot generated reports).
• By default the ID is a random UUID (uuid.uuid4()).
• Adds a --executionid option to set the ID explicitly, e.g. to share a common ID across parallel executions (pabot) or to use an application specific scheme.

*** Test Cases ***
Example
    Log    Current execution: ${EXECUTION ID}

robot --executionid 20260818-build-42 tests/

Design

This follows the design @pekkaklarck outlined in #2743: generate a UUID, allow overriding it from the command line, add it as top level suite metadata, and expose it as a global variable. Exposing the existing internal test/suite IDs as automatic variables — also proposed in that issue — is intentionally left out of scope and can be handled separately.

An explicit --metadata "Execution ID:..." still takes precedence over the automatically added metadata.

Tests & docs

• Unit tests for the new setting and execution_id property (utest/conf/test_settings.py).
• Acceptance test for the variable and the suite metadata (atest/testdata|robot/variables/builtin_variables.robot).
• User Guide: documented the new ${EXECUTION ID} automatic variable; the --executionid option is documented in --help.

Happy to adjust the variable/metadata naming, the default ID scheme, or the scope based on your feedback.