[Feature] Extract static analysis tests and add multi-template support

[ArthurCRodrigues]

Summary

The current input_output template mixes execution-based tests with static analysis (AST) tests (forbidden_import and forbidden_keyword). These static analysis tests should be migrated to a dedicated static_analysis template, which doesn't require a sandbox.

Additionally, the autograder pipeline currently only supports running a single template per assignment. It needs to be refactored to support multiple templates (accepting a list of template names), so teachers can use both input_output and static_analysis tests in the same assignment without duplicate grading runs.

Acceptance Criteria

• Create a new static_analysis template containing AST and static analysis tests.
• Remove those tests from input_output.
• Update PipelineExecution and TemplateLoaderStep to support a list of templates.
• Update CriteriaTreeService to lookup tests across all loaded templates.
• Update SandboxStep and GradeStep to check if any of the loaded templates require a sandbox.
• Ensure API and database layers do not break (e.g. support comma-separated string of templates).
• Ensure all unit tests pass with the new architecture.

[ArthurCRodrigues]

Implementation is ready for review.

• Branch: feat/issue-305-multi-template-support
• PR: [Feature] Extract static analysis tests and add multi-template support (#305) #306
• Validation: All unit tests successfully passed (pytest tests/unit/), and the template loader accurately resolves both lists and comma-separated strings of template names.