feat: Remove mellea as a dependency

[erichare]

This pull request removes the dependency on the mellea library throughout the codebase and replaces its usage with a new lightweight prompt-building utility. The changes simplify the code by eliminating custom backend shims, refactoring prompt generation to use a custom run_prompt function, and updating all affected modules and tests to use the new approach.

Dependency and Backend Removal:

• Removed mellea from project dependencies in pyproject.toml and deleted all imports and usage of mellea and related backend shims, including the SimpleBackend class and associated code (pyproject.toml, src/toolguard/buildtime/gen_py/gen_toolguards.py, src/toolguard/buildtime/gen_py/mellea_simple.py, tests/buildtime/gen_py/dependencies/test_appointment_dependencies.py, tests/buildtime/gen_py/dependencies/test_tau2_dependencies.py). [1] [2] [3] [4] [5]

Prompt Generation Refactor:

• Introduced prompt_runner.py as a lightweight replacement for mellea's generative decorator, which builds prompts from function metadata and sends them to an I_TG_LLM backend (src/toolguard/buildtime/gen_py/prompt_runner.py).
• Refactored all prompt-generating functions in prompts/gen_tests.py, prompts/improve_guard.py, and prompts/pseudo_code.py to use the new run_prompt utility instead of mellea's generative decorator. This includes updating their signatures to require an I_TG_LLM instance and moving template logic into private helper functions. [1] [2] [3] [4] [5] [6] [7]

Core Build and Dependency Logic Update:

• Updated the tool guard and dependency generation logic to remove all references to MelleaSession and instead accept an I_TG_LLM backend, propagating this change through tool_guard_generator.py, tool_dependencies.py, and related call sites. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Test Updates:

• Refactored tests to remove all usage of mellea and SimpleBackend, updating fixtures and test code to use the new prompt pipeline and I_TG_LLM directly. [1] [2] [3] [4]

Version Bump:

• Bumped the project version to 0.2.14 in pyproject.toml.